merge(3p/git): Merge git subtree at v2.29.2 r/1890

This also bumps the stable nixpkgs to 20.09 as of 2020-11-21, because
there is some breakage in the git build related to the netrc
credentials helper which someone has taken care of in nixpkgs.

The stable channel is not used for anything other than git, so this
should be fine.

Change-Id: I3575a19dab09e1e9556cf8231d717de9890484fb

1 files changed, 271 insertions, 39 deletions

diff --git a/third_party/git/Documentation/git-rebase.txt b/third_party/git/Documentation/git-rebase.txt
index 6156609cf7..38e15488f6 100644
--- a/third_party/git/Documentation/git-rebase.txt
+++ b/third_party/git/Documentation/git-rebase.txt
@@ -8,8 +8,8 @@ git-rebase - Reapply commits on top of another base tip
 SYNOPSIS
 --------
 [verse]
-'git rebase' [-i | --interactive] [<options>] [--exec <cmd>] [--onto <newbase>]
-	[<upstream> [<branch>]]
+'git rebase' [-i | --interactive] [<options>] [--exec <cmd>]
+	[--onto <newbase> | --keep-base] [<upstream> [<branch>]]
 'git rebase' [-i | --interactive] [<options>] [--exec <cmd>] [--onto <newbase>]
 	--root [<branch>]
 'git rebase' (--continue | --skip | --abort | --quit | --edit-todo | --show-current-patch)
@@ -204,6 +204,7 @@ CONFIGURATION
 -------------
 
 include::config/rebase.txt[]
+include::config/sequencer.txt[]
 
 OPTIONS
 -------
@@ -217,6 +218,24 @@ As a special case, you may use "A\...B" as a shortcut for the
 merge base of A and B if there is exactly one merge base. You can
 leave out at most one of A and B, in which case it defaults to HEAD.
 
+--keep-base::
+	Set the starting point at which to create the new commits to the
+	merge base of <upstream> <branch>. Running
+	'git rebase --keep-base <upstream> <branch>' is equivalent to
+	running 'git rebase --onto <upstream>... <upstream>'.
++
+This option is useful in the case where one is developing a feature on
+top of an upstream branch. While the feature is being worked on, the
+upstream branch may advance and it may not be the best idea to keep
+rebasing on top of the upstream but to keep the base commit as-is.
++
+Although both this option and --fork-point find the merge base between
+<upstream> and <branch>, this option uses the merge base as the _starting
+point_ on which new commits will be created, whereas --fork-point uses
+the merge base to determine the _set of commits_ which will be rebased.
++
+See also INCOMPATIBLE OPTIONS below.
+
 <upstream>::
 	Upstream branch to compare against.  May be any valid commit,
 	not just an existing branch name. Defaults to the configured
@@ -238,18 +257,79 @@ leave out at most one of A and B, in which case it defaults to HEAD.
 --quit::
 	Abort the rebase operation but HEAD is not reset back to the
 	original branch. The index and working tree are also left
-	unchanged as a result.
+	unchanged as a result. If a temporary stash entry was created
+	using --autostash, it will be saved to the stash list.
+
+--apply::
+	Use applying strategies to rebase (calling `git-am`
+	internally).  This option may become a no-op in the future
+	once the merge backend handles everything the apply one does.
++
+See also INCOMPATIBLE OPTIONS below.
 
+--empty={drop,keep,ask}::
+	How to handle commits that are not empty to start and are not
+	clean cherry-picks of any upstream commit, but which become
+	empty after rebasing (because they contain a subset of already
+	upstream changes).  With drop (the default), commits that
+	become empty are dropped.  With keep, such commits are kept.
+	With ask (implied by --interactive), the rebase will halt when
+	an empty commit is applied allowing you to choose whether to
+	drop it, edit files more, or just commit the empty changes.
+	Other options, like --exec, will use the default of drop unless
+	-i/--interactive is explicitly specified.
++
+Note that commits which start empty are kept (unless --no-keep-empty
+is specified), and commits which are clean cherry-picks (as determined
+by `git log --cherry-mark ...`) are detected and dropped as a
+preliminary step (unless --reapply-cherry-picks is passed).
++
+See also INCOMPATIBLE OPTIONS below.
+
+--no-keep-empty::
 --keep-empty::
-	Keep the commits that do not change anything from its
-	parents in the result.
+	Do not keep commits that start empty before the rebase
+	(i.e. that do not change anything from its parent) in the
+	result.  The default is to keep commits which start empty,
+	since creating such commits requires passing the --allow-empty
+	override flag to `git commit`, signifying that a user is very
+	intentionally creating such a commit and thus wants to keep
+	it.
++
+Usage of this flag will probably be rare, since you can get rid of
+commits that start empty by just firing up an interactive rebase and
+removing the lines corresponding to the commits you don't want.  This
+flag exists as a convenient shortcut, such as for cases where external
+tools generate many empty commits and you want them all removed.
++
+For commits which do not start empty but become empty after rebasing,
+see the --empty flag.
++
+See also INCOMPATIBLE OPTIONS below.
+
+--reapply-cherry-picks::
+--no-reapply-cherry-picks::
+	Reapply all clean cherry-picks of any upstream commit instead
+	of preemptively dropping them. (If these commits then become
+	empty after rebasing, because they contain a subset of already
+	upstream changes, the behavior towards them is controlled by
+	the `--empty` flag.)
++
+By default (or if `--no-reapply-cherry-picks` is given), these commits
+will be automatically dropped.  Because this necessitates reading all
+upstream commits, this can be expensive in repos with a large number
+of upstream commits that need to be read.
++
+`--reapply-cherry-picks` allows rebase to forgo reading all upstream
+commits, potentially improving performance.
 +
 See also INCOMPATIBLE OPTIONS below.
 
 --allow-empty-message::
-	By default, rebasing commits with an empty message will fail.
-	This option overrides that behavior, allowing commits with empty
-	messages to be rebased.
+	No-op.  Rebasing commits with an empty message used to fail
+	and this option would override that behavior, allowing commits
+	with empty messages to be rebased.  Now commits with an empty
+	message do not cause rebasing to halt.
 +
 See also INCOMPATIBLE OPTIONS below.
 
@@ -268,7 +348,7 @@ See also INCOMPATIBLE OPTIONS below.
 --merge::
 	Use merging strategies to rebase.  When the recursive (default) merge
 	strategy is used, this allows rebase to be aware of renames on the
-	upstream side.
+	upstream side.  This is the default.
 +
 Note that a rebase merge works by replaying each commit from the working
 branch on top of the <upstream> branch.  Because of this, when a merge
@@ -307,9 +387,12 @@ See also INCOMPATIBLE OPTIONS below.
 
 -S[<keyid>]::
 --gpg-sign[=<keyid>]::
+--no-gpg-sign::
 	GPG-sign commits. The `keyid` argument is optional and
 	defaults to the committer identity; if specified, it must be
-	stuck to the option without a space.
+	stuck to the option without a space. `--no-gpg-sign` is useful to
+	countermand both `commit.gpgSign` configuration variable, and
+	earlier `--gpg-sign`.
 
 -q::
 --quiet::
@@ -338,7 +421,7 @@ See also INCOMPATIBLE OPTIONS below.
 	Ensure at least <n> lines of surrounding context match before
 	and after each change.  When fewer lines of surrounding
 	context exist they all must match.  By default no context is
-	ever ignored.
+	ever ignored.  Implies --apply.
 +
 See also INCOMPATIBLE OPTIONS below.
 
@@ -367,20 +450,48 @@ When --fork-point is active, 'fork_point' will be used instead of
 <branch>` command (see linkgit:git-merge-base[1]).  If 'fork_point'
 ends up being empty, the <upstream> will be used as a fallback.
 +
-If either <upstream> or --root is given on the command line, then the
-default is `--no-fork-point`, otherwise the default is `--fork-point`.
+If <upstream> is given on the command line, then the default is
+`--no-fork-point`, otherwise the default is `--fork-point`.
++
+If your branch was based on <upstream> but <upstream> was rewound and
+your branch contains commits which were dropped, this option can be used
+with `--keep-base` in order to drop those commits from your branch.
++
+See also INCOMPATIBLE OPTIONS below.
 
 --ignore-whitespace::
+	Ignore whitespace differences when trying to reconcile
+differences. Currently, each backend implements an approximation of
+this behavior:
++
+apply backend: When applying a patch, ignore changes in whitespace in
+context lines. Unfortunately, this means that if the "old" lines being
+replaced by the patch differ only in whitespace from the existing
+file, you will get a merge conflict instead of a successful patch
+application.
++
+merge backend: Treat lines with only whitespace changes as unchanged
+when merging. Unfortunately, this means that any patch hunks that were
+intended to modify whitespace and nothing else will be dropped, even
+if the other side had no changes that conflicted.
+
 --whitespace=<option>::
-	These flag are passed to the 'git apply' program
+	This flag is passed to the 'git apply' program
 	(see linkgit:git-apply[1]) that applies the patch.
+	Implies --apply.
 +
 See also INCOMPATIBLE OPTIONS below.
 
 --committer-date-is-author-date::
+	Instead of using the current time as the committer date, use
+	the author date of the commit being rebased as the committer
+	date. This option implies `--force-rebase`.
+
 --ignore-date::
-	These flags are passed to 'git am' to easily change the dates
-	of the rebased commits (see linkgit:git-am[1]).
+--reset-author-date::
+	Instead of using the author date of the original commit, use
+	the current time as the	author date of the rebased commit.  This
+	option implies `--force-rebase`.
 +
 See also INCOMPATIBLE OPTIONS below.
 
@@ -421,8 +532,8 @@ the `rebase-cousins` mode is turned on, such commits are instead rebased
 onto `<upstream>` (or `<onto>`, if specified).
 +
 The `--rebase-merges` mode is similar in spirit to the deprecated
-`--preserve-merges`, but in contrast to that option works well in interactive
-rebases: commits can be reordered, inserted and dropped at will.
+`--preserve-merges` but works with interactive rebases,
+where commits can be reordered, inserted and dropped at will.
 +
 It is currently only possible to recreate the merge commits using the
 `recursive` merge strategy; Different merge strategies can be used only via
@@ -517,10 +628,8 @@ INCOMPATIBLE OPTIONS
 
 The following options:
 
- * --committer-date-is-author-date
- * --ignore-date
+ * --apply
  * --whitespace
- * --ignore-whitespace
  * -C
 
 are incompatible with the following options:
@@ -534,7 +643,9 @@ are incompatible with the following options:
  * --preserve-merges
  * --interactive
  * --exec
- * --keep-empty
+ * --no-keep-empty
+ * --empty=
+ * --reapply-cherry-picks
  * --edit-todo
  * --root when used in combination with --onto
 
@@ -543,33 +654,152 @@ In addition, the following pairs of options are incompatible:
  * --preserve-merges and --interactive
  * --preserve-merges and --signoff
  * --preserve-merges and --rebase-merges
- * --rebase-merges and --strategy
- * --rebase-merges and --strategy-option
+ * --preserve-merges and --empty=
+ * --preserve-merges and --ignore-whitespace
+ * --preserve-merges and --committer-date-is-author-date
+ * --preserve-merges and --ignore-date
+ * --keep-base and --onto
+ * --keep-base and --root
+ * --fork-point and --root
 
 BEHAVIORAL DIFFERENCES
 -----------------------
 
-There are some subtle differences how the backends behave.
+git rebase has two primary backends: apply and merge.  (The apply
+backend used to be known as the 'am' backend, but the name led to
+confusion as it looks like a verb instead of a noun.  Also, the merge
+backend used to be known as the interactive backend, but it is now
+used for non-interactive cases as well.  Both were renamed based on
+lower-level functionality that underpinned each.) There are some
+subtle differences in how these two backends behave:
 
 Empty commits
 ~~~~~~~~~~~~~
 
-The am backend drops any "empty" commits, regardless of whether the
-commit started empty (had no changes relative to its parent to
-start with) or ended empty (all changes were already applied
-upstream in other commits).
+The apply backend unfortunately drops intentionally empty commits, i.e.
+commits that started empty, though these are rare in practice.  It
+also drops commits that become empty and has no option for controlling
+this behavior.
+
+The merge backend keeps intentionally empty commits by default (though
+with -i they are marked as empty in the todo list editor, or they can
+be dropped automatically with --no-keep-empty).
 
-The interactive backend drops commits by default that
-started empty and halts if it hits a commit that ended up empty.
-The `--keep-empty` option exists for the interactive backend to allow
-it to keep commits that started empty.
+Similar to the apply backend, by default the merge backend drops
+commits that become empty unless -i/--interactive is specified (in
+which case it stops and asks the user what to do).  The merge backend
+also has an --empty={drop,keep,ask} option for changing the behavior
+of handling commits that become empty.
 
 Directory rename detection
 ~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-Directory rename heuristics are enabled in the merge and interactive
-backends.  Due to the lack of accurate tree information, directory
-rename detection is disabled in the am backend.
+Due to the lack of accurate tree information (arising from
+constructing fake ancestors with the limited information available in
+patches), directory rename detection is disabled in the apply backend.
+Disabled directory rename detection means that if one side of history
+renames a directory and the other adds new files to the old directory,
+then the new files will be left behind in the old directory without
+any warning at the time of rebasing that you may want to move these
+files into the new directory.
+
+Directory rename detection works with the merge backend to provide you
+warnings in such cases.
+
+Context
+~~~~~~~
+
+The apply backend works by creating a sequence of patches (by calling
+`format-patch` internally), and then applying the patches in sequence
+(calling `am` internally).  Patches are composed of multiple hunks,
+each with line numbers, a context region, and the actual changes.  The
+line numbers have to be taken with some fuzz, since the other side
+will likely have inserted or deleted lines earlier in the file.  The
+context region is meant to help find how to adjust the line numbers in
+order to apply the changes to the right lines.  However, if multiple
+areas of the code have the same surrounding lines of context, the
+wrong one can be picked.  There are real-world cases where this has
+caused commits to be reapplied incorrectly with no conflicts reported.
+Setting diff.context to a larger value may prevent such types of
+problems, but increases the chance of spurious conflicts (since it
+will require more lines of matching context to apply).
+
+The merge backend works with a full copy of each relevant file,
+insulating it from these types of problems.
+
+Labelling of conflicts markers
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When there are content conflicts, the merge machinery tries to
+annotate each side's conflict markers with the commits where the
+content came from.  Since the apply backend drops the original
+information about the rebased commits and their parents (and instead
+generates new fake commits based off limited information in the
+generated patches), those commits cannot be identified; instead it has
+to fall back to a commit summary.  Also, when merge.conflictStyle is
+set to diff3, the apply backend will use "constructed merge base" to
+label the content from the merge base, and thus provide no information
+about the merge base commit whatsoever.
+
+The merge backend works with the full commits on both sides of history
+and thus has no such limitations.
+
+Hooks
+~~~~~
+
+The apply backend has not traditionally called the post-commit hook,
+while the merge backend has.  Both have called the post-checkout hook,
+though the merge backend has squelched its output.  Further, both
+backends only call the post-checkout hook with the starting point
+commit of the rebase, not the intermediate commits nor the final
+commit.  In each case, the calling of these hooks was by accident of
+implementation rather than by design (both backends were originally
+implemented as shell scripts and happened to invoke other commands
+like 'git checkout' or 'git commit' that would call the hooks).  Both
+backends should have the same behavior, though it is not entirely
+clear which, if any, is correct.  We will likely make rebase stop
+calling either of these hooks in the future.
+
+Interruptability
+~~~~~~~~~~~~~~~~
+
+The apply backend has safety problems with an ill-timed interrupt; if
+the user presses Ctrl-C at the wrong time to try to abort the rebase,
+the rebase can enter a state where it cannot be aborted with a
+subsequent `git rebase --abort`.  The merge backend does not appear to
+suffer from the same shortcoming.  (See
+https://lore.kernel.org/git/20200207132152.GC2868@szeder.dev/ for
+details.)
+
+Commit Rewording
+~~~~~~~~~~~~~~~~
+
+When a conflict occurs while rebasing, rebase stops and asks the user
+to resolve.  Since the user may need to make notable changes while
+resolving conflicts, after conflicts are resolved and the user has run
+`git rebase --continue`, the rebase should open an editor and ask the
+user to update the commit message.  The merge backend does this, while
+the apply backend blindly applies the original commit message.
+
+Miscellaneous differences
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+There are a few more behavioral differences that most folks would
+probably consider inconsequential but which are mentioned for
+completeness:
+
+* Reflog: The two backends will use different wording when describing
+  the changes made in the reflog, though both will make use of the
+  word "rebase".
+
+* Progress, informational, and error messages: The two backends
+  provide slightly different progress and informational messages.
+  Also, the apply backend writes error messages (such as "Your files
+  would be overwritten...") to stdout, while the merge backend writes
+  them to stderr.
+
+* State directories: The two backends keep their state in different
+  directories under .git/
 
 include::merge-strategies.txt[]
 
@@ -832,7 +1062,8 @@ Hard case: The changes are not the same.::
 	This happens if the 'subsystem' rebase had conflicts, or used
 	`--interactive` to omit, edit, squash, or fixup commits; or
 	if the upstream used one of `commit --amend`, `reset`, or
-	`filter-branch`.
+	a full history rewriting command like
+	https://github.com/newren/git-filter-repo[`filter-repo`].
 
 
 The easy case
@@ -843,7 +1074,8 @@ Only works if the changes (patch IDs based on the diff contents) on
 'subsystem' did.
 
 In that case, the fix is easy because 'git rebase' knows to skip
-changes that are already present in the new upstream.  So if you say
+changes that are already present in the new upstream (unless
+`--reapply-cherry-picks` is given). So if you say
 (assuming you're on 'topic')
 ------------
     $ git rebase subsystem
@@ -870,7 +1102,7 @@ NOTE: While an "easy case recovery" sometimes appears to be successful
       --interactive` will be **resurrected**!
 
 The idea is to manually tell 'git rebase' "where the old 'subsystem'
-ended and your 'topic' began", that is, what the old merge-base
+ended and your 'topic' began", that is, what the old merge base
 between them was.  You will have to find a way to name the last commit
 of the old 'subsystem', for example: