merge(third_party/git): Merge squashed git subtree at v2.23.0 r/373

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+Git 2.9 Release Notes
+=====================
+
+Backward compatibility notes
+----------------------------
+
+The end-user facing Porcelain level commands in the "git diff" and
+"git log" family by default enable the rename detection; you can still
+use "diff.renames" configuration variable to disable this.
+
+Merging two branches that have no common ancestor with "git merge" is
+by default forbidden now to prevent creating such an unusual merge by
+mistake.
+
+The output formats of "git log" that indents the commit log message by
+4 spaces now expands HT in the log message by default.  You can use
+the "--no-expand-tabs" option to disable this.
+
+"git commit-tree" plumbing command required the user to always sign
+its result when the user sets the commit.gpgsign configuration
+variable, which was an ancient mistake, which this release corrects.
+A script that drives commit-tree, if it relies on this mistake, now
+needs to read commit.gpgsign and pass the -S option as necessary.
+
+
+Updates since v2.8
+------------------
+
+UI, Workflows & Features
+
+ * Comes with git-multimail 1.3.1 (in contrib/).
+
+ * The end-user facing commands like "git diff" and "git log"
+   now enable the rename detection by default.
+
+ * The credential.helper configuration variable is cumulative and
+   there is no good way to override it from the command line.  As
+   a special case, giving an empty string as its value now serves
+   as the signal to clear the values specified in various files.
+
+ * A new "interactive.diffFilter" configuration can be used to
+   customize the diff shown in "git add -i" sessions.
+
+ * "git p4" now allows P4 author names to be mapped to Git author
+   names.
+
+ * "git rebase -x" can be used without passing "-i" option.
+
+ * "git -c credential.<var>=<value> submodule" can now be used to
+   propagate configuration variables related to credential helper
+   down to the submodules.
+
+ * "git tag" can create an annotated tag without explicitly given an
+   "-a" (or "-s") option (i.e. when a tag message is given).  A new
+   configuration variable, tag.forceSignAnnotated, can be used to tell
+   the command to create signed tag in such a situation.
+
+ * "git merge" used to allow merging two branches that have no common
+   base by default, which led to a brand new history of an existing
+   project created and then get pulled by an unsuspecting maintainer,
+   which allowed an unnecessary parallel history merged into the
+   existing project.  The command has been taught not to allow this by
+   default, with an escape hatch "--allow-unrelated-histories" option
+   to be used in a rare event that merges histories of two projects
+   that started their lives independently.
+
+ * "git pull" has been taught to pass the "--allow-unrelated-histories"
+   option to underlying "git merge".
+
+ * "git apply -v" learned to report paths in the patch that were
+   skipped via --include/--exclude mechanism or being outside the
+   current working directory.
+
+ * Shell completion (in contrib/) updates.
+
+ * The commit object name reported when "rebase -i" stops has been
+   shortened.
+
+ * "git worktree add" can be given "--no-checkout" option to only
+   create an empty worktree without checking out the files.
+
+ * "git mergetools" learned to drive ExamDiff.
+
+ * "git pull --rebase" learned "--[no-]autostash" option, so that
+   the rebase.autostash configuration variable set to true can be
+   overridden from the command line.
+
+ * When "git log" shows the log message indented by 4-spaces, the
+   remainder of a line after a HT does not align in the way the author
+   originally intended.  The command now expands tabs by default to help
+   such a case, and allows the users to override it with a new option,
+   "--no-expand-tabs".
+
+ * "git send-email" now uses a more readable timestamps when
+   formulating a message ID.
+
+ * "git rerere" can encounter two or more files with the same conflict
+   signature that have to be resolved in different ways, but there was
+   no way to record these separate resolutions.
+
+ * "git p4" learned to record P4 jobs in Git commit that imports from
+   the history in Perforce.
+
+ * "git describe --contains" often made a hard-to-justify choice of
+   tag to name a given commit, because it tried to come up
+   with a name with smallest number of hops from a tag, causing an old
+   commit whose close descendant that is recently tagged were not
+   described with respect to an old tag but with a newer tag.  It did
+   not help that its computation of "hop" count was further tweaked to
+   penalize being on a side branch of a merge.  The logic has been
+   updated to favor using the tag with the oldest tagger date, which
+   is a lot easier to explain to the end users: "We describe a commit
+   in terms of the (chronologically) oldest tag that contains the
+   commit."
+
+ * "git clone" learned the "--shallow-submodules" option.
+
+ * HTTP transport clients learned to throw extra HTTP headers at the
+   server, specified via http.extraHeader configuration variable.
+
+ * The "--compaction-heuristic" option to "git diff" family of
+   commands enables a heuristic to make the patch output more readable
+   by using a blank line as a strong hint that the contents before and
+   after it belong to logically separate units.  It is still
+   experimental.
+
+ * A new configuration variable core.hooksPath allows customizing
+   where the hook directory is.
+
+ * An earlier addition of "sanitize_submodule_env" with 14111fc4 (git:
+   submodule honor -c credential.* from command line, 2016-02-29)
+   turned out to be a convoluted no-op; implement what it wanted to do
+   correctly, and stop filtering settings given via "git -c var=val".
+
+ * "git commit --dry-run" reported "No, no, you cannot commit." in one
+   case where "git commit" would have allowed you to commit, and this
+   improves it a little bit ("git commit --dry-run --short" still does
+   not give you the correct answer, for example).  This is a stop-gap
+   measure in that "commit --short --dry-run" still gives an incorrect
+   result.
+
+ * The experimental "multiple worktree" feature gains more safety to
+   forbid operations on a branch that is checked out or being actively
+   worked on elsewhere, by noticing that e.g. it is being rebased.
+
+ * "git format-patch" learned a new "--base" option to record what
+   (public, well-known) commit the original series was built on in
+   its output.
+
+ * "git commit" learned to pay attention to the "commit.verbose"
+   configuration variable and act as if the "--verbose" option
+   was given from the command line.
+
+ * Updated documentation gives hints to GMail users with two-factor
+   auth enabled that they need app-specific-password when using
+   "git send-email".
+
+ * The manpage output of our documentation did not render well in
+   terminal; typeset literals in bold by default to make them stand
+   out more.
+
+ * The mark-up in the top-level README.md file has been updated to
+   typeset CLI command names differently from the body text.
+
+
+Performance, Internal Implementation, Development Support etc.
+
+ * The embedded args argv-array in the child process is used to build
+   the command line to run pack-objects instead of using a separate
+   array of strings.
+
+ * A test for tags has been restructured so that more parts of it can
+   easily be run on a platform without a working GnuPG.
+
+ * The startup_info data, which records if we are working inside a
+   repository (among other things), are now uniformly available to Git
+   subcommand implementations, and Git avoids attempting to touch
+   references when we are not in a repository.
+
+ * The command line argument parser for "receive-pack" has been
+   rewritten to use parse-options.
+
+ * A major part of "git submodule update" has been ported to C to take
+   advantage of the recently added framework to run download tasks in
+   parallel.  Other updates to "git submodule" that move pieces of
+   logic to C continues.
+
+ * Rename bunch of tests on "git clone" for better organization.
+
+ * The tests that involve running httpd leaked the system-wide
+   configuration in /etc/gitconfig to the tested environment.
+
+ * Build updates for MSVC.
+
+ * The repository set-up sequence has been streamlined (the biggest
+   change is that there is no longer git_config_early()), so that we
+   do not attempt to look into refs/* when we know we do not have a
+   Git repository.
+
+ * Code restructuring around the "refs" API to prepare for pluggable
+   refs backends.
+
+ * Sources to many test helper binaries and the generated helpers
+   have been moved to t/helper/ subdirectory to reduce clutter at the
+   top level of the tree.
+
+ * Unify internal logic between "git tag -v" and "git verify-tag"
+   commands by making one directly call into the other.
+
+ * "merge-recursive" strategy incorrectly checked if a path that is
+   involved in its internal merge exists in the working tree.
+
+ * The test scripts for "git p4" (but not "git p4" implementation
+   itself) has been updated so that they would work even on a system
+   where the installed version of Python is python 3.
+
+ * As nobody maintains our in-tree git.spec.in and distros use their
+   own spec file, we stopped pretending that we support "make rpm".
+
+ * Move from "unsigned char[20]" to "struct object_id" continues.
+
+ * The code for warning_errno/die_errno has been refactored and a new
+   error_errno() reporting helper is introduced.
+   (merge 1da045f nd/error-errno later to maint).
+
+ * Running tests with '-x' option to trace the individual command
+   executions is a useful way to debug test scripts, but some tests
+   that capture the standard error stream and check what the command
+   said can be broken with the trace output mixed in.  When running
+   our tests under "bash", however, we can redirect the trace output
+   to another file descriptor to keep the standard error of programs
+   being tested intact.
+
+ * t0040 had too many unnecessary repetitions in its test data.  Teach
+   test-parse-options program so that a caller can tell what it
+   expects in its output, so that these repetitions can be cleaned up.
+
+ * Add perf test for "rebase -i".
+
+ * Common mistakes when writing gitlink: in our documentation are
+   found by "make check-docs".
+
+ * t9xxx series has been updated primarily for readability, while
+   fixing small bugs in it.  A few scripted Porcelain commands have
+   also been updated to fix possible bugs around their use of
+   "test -z" and "test -n".
+
+ * CI test was taught to run git-svn tests.
+
+ * "git cat-file --batch-all" has been sped up, by taking advantage
+   of the fact that it does not have to read a list of objects, in two
+   ways.
+
+ * test updates to make it more readable and maintainable.
+   (merge e6273f4 es/t1500-modernize later to maint).
+
+ * "make DEVELOPER=1" worked as expected; setting DEVELOPER=1 in
+   config.mak didn't.
+   (merge 51dd3e8 mm/makefile-developer-can-be-in-config-mak later to maint).
+
+ * The way how "submodule--helper list" signals unmatch error to its
+   callers has been updated.
+
+ * A bash-ism "local" has been removed from "git submodule" scripted
+   Porcelain.
+
+
+Also contains various documentation updates and code clean-ups.
+
+
+Fixes since v2.8
+----------------
+
+Unless otherwise noted, all the fixes since v2.8 in the maintenance
+track are contained in this release (see the maintenance releases'
+notes for details).
+
+ * "git config --get-urlmatch", unlike other variants of the "git
+   config --get" family, did not signal error with its exit status
+   when there was no matching configuration.
+
+ * The "--local-env-vars" and "--resolve-git-dir" options of "git
+   rev-parse" failed to work outside a repository when the command's
+   option parsing was rewritten in 1.8.5 era.
+
+ * "git index-pack --keep[=<msg>] pack-$name.pack" simply did not work.
+
+ * Fetching of history by naming a commit object name directly didn't
+   work across remote-curl transport.
+
+ * A small memory leak in an error codepath has been plugged in xdiff
+   code.
+
+ * strbuf_getwholeline() did not NUL-terminate the buffer on certain
+   corner cases in its error codepath.
+
+ * "git mergetool" did not work well with conflicts that both sides
+   deleted.
+
+ * "git send-email" had trouble parsing alias file in mailrc format
+   when lines in it had trailing whitespaces on them.
+
+ * When "git merge --squash" stopped due to conflict, the concluding
+   "git commit" failed to read in the SQUASH_MSG that shows the log
+   messages from all the squashed commits.
+
+ * "git merge FETCH_HEAD" dereferenced NULL pointer when merging
+   nothing into an unborn history (which is arguably unusual usage,
+   which perhaps was the reason why nobody noticed it).
+
+ * When "git worktree" feature is in use, "git branch -d" allowed
+   deletion of a branch that is checked out in another worktree,
+   which was wrong.
+
+ * When "git worktree" feature is in use, "git branch -m" renamed a
+   branch that is checked out in another worktree without adjusting
+   the HEAD symbolic ref for the worktree.
+
+ * "git diff -M" used to work better when two originally identical
+   files A and B got renamed to X/A and X/B by pairing A to X/A and B
+   to X/B, but this was broken in the 2.0 timeframe.
+
+ * "git send-pack --all <there>" was broken when its command line
+   option parsing was written in the 2.6 timeframe.
+
+ * "git format-patch --help" showed `-s` and `--no-patch` as if these
+   are valid options to the command.  We already hide `--patch` option
+   from the documentation, because format-patch is about showing the
+   diff, and the documentation now hides these options as well.
+
+ * When running "git blame $path" with unnormalized data in the index
+   for the path, the data in the working tree was blamed, even though
+   "git add" would not have changed what is already in the index, due
+   to "safe crlf" that disables the line-end conversion.  It has been
+   corrected.
+
+ * A change back in version 2.7 to "git branch" broke display of a
+   symbolic ref in a non-standard place in the refs/ hierarchy (we
+   expect symbolic refs to appear in refs/remotes/*/HEAD to point at
+   the primary branch the remote has, and as .git/HEAD to point at the
+   branch we locally checked out).
+
+ * A partial rewrite of "git submodule" in the 2.7 timeframe changed
+   the way the gitdir: pointer in the submodules point at the real
+   repository location to use absolute paths by accident.  This has
+   been corrected.
+
+ * "git commit" misbehaved in a few minor ways when an empty message
+   is given via -m '', all of which has been corrected.
+
+ * Support for CRAM-MD5 authentication method in "git imap-send" did
+   not work well.
+
+ * Upcoming OpenSSL 1.1.0 will break compilation by updating a few API
+   elements we use in imap-send, which has been adjusted for the change.
+
+ * The socks5:// proxy support added back in 2.6.4 days was not aware
+   that socks5h:// proxies behave differently from socks5:// proxies.
+
+ * "git config" had a codepath that tried to pass a NULL to
+   printf("%s"), which nobody seems to have noticed.
+
+ * On Cygwin, object creation uses the "create a temporary and then
+   rename it to the final name" pattern, not "create a temporary,
+   hardlink it to the final name and then unlink the temporary"
+   pattern.
+
+   This is necessary to use Git on Windows shared directories, and is
+   already enabled for the MinGW and plain Windows builds.  It also
+   has been used in Cygwin packaged versions of Git for quite a while.
+   See http://thread.gmane.org/gmane.comp.version-control.git/291853
+
+ * "merge-octopus" strategy did not ensure that the index is clean
+   when merge begins.
+
+ * When "git merge" notices that the merge can be resolved purely at
+   the tree level (without having to merge blobs) and the resulting
+   tree happens to already exist in the object store, it forgot to
+   update the index, which left an inconsistent state that would
+   break later operations.
+
+ * "git submodule" reports the paths of submodules the command
+   recurses into, but these paths were incorrectly reported when
+   the command was not run from the root level of the superproject.
+
+ * The "user.useConfigOnly" configuration variable makes it an error
+   if users do not explicitly set user.name and user.email.  However,
+   its check was not done early enough and allowed another error to
+   trigger, reporting that the default value we guessed from the
+   system setting was unusable.  This was a suboptimal end-user
+   experience as we want the users to set user.name/user.email without
+   relying on the auto-detection at all.
+
+ * "git mv old new" did not adjust the path for a submodule that lives
+   as a subdirectory inside old/ directory correctly.
+
+ * "git replace -e" did not honour "core.editor" configuration.
+
+ * "git push" from a corrupt repository that attempts to push a large
+   number of refs deadlocked; the thread to relay rejection notices
+   for these ref updates blocked on writing them to the main thread,
+   after the main thread at the receiving end notices that the push
+   failed and decides not to read these notices and return a failure.
+
+ * mmap emulation on Windows has been optimized and work better without
+   consuming paging store when not needed.
+
+ * A question by "git send-email" to ask the identity of the sender
+   has been updated.
+
+ * UI consistency improvements for "git mergetool".
+
+ * "git rebase -m" could be asked to rebase an entire branch starting
+   from the root, but failed by assuming that there always is a parent
+   commit to the first commit on the branch.
+
+ * Fix a broken "p4 lfs" test.
+
+ * Recent update to Git LFS broke "git p4" by changing the output from
+   its "lfs pointer" subcommand.
+
+ * "git fetch" test t5510 was flaky while running a (forced) automagic
+   garbage collection.
+
+ * Documentation updates to help contributors setting up Travis CI
+   test for their patches.
+
+ * Some multi-byte encoding can have a backslash byte as a later part
+   of one letter, which would confuse "highlight" filter used in
+   gitweb.
+
+ * "git commit-tree" plumbing command required the user to always sign
+   its result when the user sets the commit.gpgsign configuration
+   variable, which was an ancient mistake.  Rework "git rebase" that
+   relied on this mistake so that it reads commit.gpgsign and pass (or
+   not pass) the -S option to "git commit-tree" to keep the end-user
+   expectation the same, while teaching "git commit-tree" to ignore
+   the configuration variable.  This will stop requiring the users to
+   sign commit objects used internally as an implementation detail of
+   "git stash".
+
+ * "http.cookieFile" configuration variable clearly wants a pathname,
+   but we forgot to treat it as such by e.g. applying tilde expansion.
+
+ * Consolidate description of tilde-expansion that is done to
+   configuration variables that take pathname to a single place.
+
+ * Correct faulty recommendation to use "git submodule deinit ." when
+   de-initialising all submodules, which would result in a strange
+   error message in a pathological corner case.
+
+ * Many 'linkgit:<git documentation page>' references were broken,
+   which are all fixed with this.
+
+ * "git rerere" can get confused by conflict markers deliberately left
+   by the inner merge step, because they are indistinguishable from
+   the real conflict markers left by the outermost merge which are
+   what the end user and "rerere" need to look at.  This was fixed by
+   making the conflict markers left by the inner merges a bit longer.
+   (merge 0f9fd5c jc/ll-merge-internal later to maint).
+
+ * CI test was taught to build documentation pages.
+
+ * "git fsck" learned to catch NUL byte in a commit object as
+   potential error and warn.
+
+ * Portability enhancement for "rebase -i" to help platforms whose
+   shell does not like "for i in <empty>" (which is not POSIX-kosher).
+
+ * On Windows, .git and optionally any files whose name starts with a
+   dot are now marked as hidden, with a core.hideDotFiles knob to
+   customize this behaviour.
+
+ * Documentation for "git merge --verify-signatures" has been updated
+   to clarify that the signature of only the commit at the tip is
+   verified.  Also the phrasing used for signature and key validity is
+   adjusted to align with that used by OpenPGP.
+
+ * A couple of bugs around core.autocrlf have been fixed.
+
+ * Many commands normalize command line arguments from NFD to NFC
+   variant of UTF-8 on OSX, but commands in the "diff" family did
+   not, causing "git diff $path" to complain that no such path is
+   known to Git.  They have been taught to do the normalization.
+
+ * "git difftool" learned to handle unmerged paths correctly in
+   dir-diff mode.
+
+ * The "are we talking with TTY, doing an interactive session?"
+   detection has been updated to work better for "Git for Windows".
+
+ * We forgot to add "git log --decorate=auto" to documentation when we
+   added the feature back in v2.1.0 timeframe.
+   (merge 462cbb4 rj/log-decorate-auto later to maint).
+
+ * "git fast-import --export-marks" would overwrite the existing marks
+   file even when it makes a dump from its custom die routine.
+   Prevent it from doing so when we have an import-marks file but
+   haven't finished reading it.
+   (merge f4beed6 fc/fast-import-broken-marks-file later to maint).
+
+ * "git rebase -i", after it fails to auto-resolve the conflict, had
+   an unnecessary call to "git rerere" from its very early days, which
+   was spotted recently; the call has been removed.
+   (merge 7063693 js/rebase-i-dedup-call-to-rerere later to maint).
+
+ * Other minor clean-ups and documentation updates
+   (merge cd82b7a pa/cherry-pick-doc-typo later to maint).
+   (merge 2bb73ae rs/patch-id-use-skip-prefix later to maint).
+   (merge aa20cbc rs/apply-name-terminate later to maint).
+   (merge fe17fc0 jc/t2300-setup later to maint).
+   (merge e256eec jk/shell-portability later to maint).