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

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+git-read-tree(1)
+================
+
+NAME
+----
+git-read-tree - Reads tree information into the index
+
+
+SYNOPSIS
+--------
+[verse]
+'git read-tree' [[-m [--trivial] [--aggressive] | --reset | --prefix=<prefix>]
+		[-u [--exclude-per-directory=<gitignore>] | -i]]
+		[--index-output=<file>] [--no-sparse-checkout]
+		(--empty | <tree-ish1> [<tree-ish2> [<tree-ish3>]])
+
+
+DESCRIPTION
+-----------
+Reads the tree information given by <tree-ish> into the index,
+but does not actually *update* any of the files it "caches". (see:
+linkgit:git-checkout-index[1])
+
+Optionally, it can merge a tree into the index, perform a
+fast-forward (i.e. 2-way) merge, or a 3-way merge, with the `-m`
+flag.  When used with `-m`, the `-u` flag causes it to also update
+the files in the work tree with the result of the merge.
+
+Trivial merges are done by 'git read-tree' itself.  Only conflicting paths
+will be in unmerged state when 'git read-tree' returns.
+
+OPTIONS
+-------
+-m::
+	Perform a merge, not just a read.  The command will
+	refuse to run if your index file has unmerged entries,
+	indicating that you have not finished previous merge you
+	started.
+
+--reset::
+	Same as -m, except that unmerged entries are discarded instead
+	of failing. When used with `-u`, updates leading to loss of
+	working tree changes will not abort the operation.
+
+-u::
+	After a successful merge, update the files in the work
+	tree with the result of the merge.
+
+-i::
+	Usually a merge requires the index file as well as the
+	files in the working tree to be up to date with the
+	current head commit, in order not to lose local
+	changes.  This flag disables the check with the working
+	tree and is meant to be used when creating a merge of
+	trees that are not directly related to the current
+	working tree status into a temporary index file.
+
+-n::
+--dry-run::
+	Check if the command would error out, without updating the index
+	or the files in the working tree for real.
+
+-v::
+	Show the progress of checking files out.
+
+--trivial::
+	Restrict three-way merge by 'git read-tree' to happen
+	only if there is no file-level merging required, instead
+	of resolving merge for trivial cases and leaving
+	conflicting files unresolved in the index.
+
+--aggressive::
+	Usually a three-way merge by 'git read-tree' resolves
+	the merge for really trivial cases and leaves other
+	cases unresolved in the index, so that porcelains can
+	implement different merge policies.  This flag makes the
+	command resolve a few more cases internally:
++
+* when one side removes a path and the other side leaves the path
+  unmodified.  The resolution is to remove that path.
+* when both sides remove a path.  The resolution is to remove that path.
+* when both sides add a path identically.  The resolution
+  is to add that path.
+
+--prefix=<prefix>::
+	Keep the current index contents, and read the contents
+	of the named tree-ish under the directory at `<prefix>`.
+	The command will refuse to overwrite entries that already
+	existed in the original index file.
+
+--exclude-per-directory=<gitignore>::
+	When running the command with `-u` and `-m` options, the
+	merge result may need to overwrite paths that are not
+	tracked in the current branch.  The command usually
+	refuses to proceed with the merge to avoid losing such a
+	path.  However this safety valve sometimes gets in the
+	way.  For example, it often happens that the other
+	branch added a file that used to be a generated file in
+	your branch, and the safety valve triggers when you try
+	to switch to that branch after you ran `make` but before
+	running `make clean` to remove the generated file.  This
+	option tells the command to read per-directory exclude
+	file (usually '.gitignore') and allows such an untracked
+	but explicitly ignored file to be overwritten.
+
+--index-output=<file>::
+	Instead of writing the results out to `$GIT_INDEX_FILE`,
+	write the resulting index in the named file.  While the
+	command is operating, the original index file is locked
+	with the same mechanism as usual.  The file must allow
+	to be rename(2)ed into from a temporary file that is
+	created next to the usual index file; typically this
+	means it needs to be on the same filesystem as the index
+	file itself, and you need write permission to the
+	directories the index file and index output file are
+	located in.
+
+--[no-]recurse-submodules::
+	Using --recurse-submodules will update the content of all initialized
+	submodules according to the commit recorded in the superproject by
+	calling read-tree recursively, also setting the submodules HEAD to be
+	detached at that commit.
+
+--no-sparse-checkout::
+	Disable sparse checkout support even if `core.sparseCheckout`
+	is true.
+
+--empty::
+	Instead of reading tree object(s) into the index, just empty
+	it.
+
+-q::
+--quiet::
+	Quiet, suppress feedback messages.
+
+<tree-ish#>::
+	The id of the tree object(s) to be read/merged.
+
+
+MERGING
+-------
+If `-m` is specified, 'git read-tree' can perform 3 kinds of
+merge, a single tree merge if only 1 tree is given, a
+fast-forward merge with 2 trees, or a 3-way merge if 3 or more trees are
+provided.
+
+
+Single Tree Merge
+~~~~~~~~~~~~~~~~~
+If only 1 tree is specified, 'git read-tree' operates as if the user did not
+specify `-m`, except that if the original index has an entry for a
+given pathname, and the contents of the path match with the tree
+being read, the stat info from the index is used. (In other words, the
+index's stat()s take precedence over the merged tree's).
+
+That means that if you do a `git read-tree -m <newtree>` followed by a
+`git checkout-index -f -u -a`, the 'git checkout-index' only checks out
+the stuff that really changed.
+
+This is used to avoid unnecessary false hits when 'git diff-files' is
+run after 'git read-tree'.
+
+
+Two Tree Merge
+~~~~~~~~~~~~~~
+
+Typically, this is invoked as `git read-tree -m $H $M`, where $H
+is the head commit of the current repository, and $M is the head
+of a foreign tree, which is simply ahead of $H (i.e. we are in a
+fast-forward situation).
+
+When two trees are specified, the user is telling 'git read-tree'
+the following:
+
+     1. The current index and work tree is derived from $H, but
+	the user may have local changes in them since $H.
+
+     2. The user wants to fast-forward to $M.
+
+In this case, the `git read-tree -m $H $M` command makes sure
+that no local change is lost as the result of this "merge".
+Here are the "carry forward" rules, where "I" denotes the index,
+"clean" means that index and work tree coincide, and "exists"/"nothing"
+refer to the presence of a path in the specified commit:
+
+....
+	I                   H        M        Result
+       -------------------------------------------------------
+     0  nothing             nothing  nothing  (does not happen)
+     1  nothing             nothing  exists   use M
+     2  nothing             exists   nothing  remove path from index
+     3  nothing             exists   exists,  use M if "initial checkout",
+				     H == M   keep index otherwise
+				     exists,  fail
+				     H != M
+
+        clean I==H  I==M
+       ------------------
+     4  yes   N/A   N/A     nothing  nothing  keep index
+     5  no    N/A   N/A     nothing  nothing  keep index
+
+     6  yes   N/A   yes     nothing  exists   keep index
+     7  no    N/A   yes     nothing  exists   keep index
+     8  yes   N/A   no      nothing  exists   fail
+     9  no    N/A   no      nothing  exists   fail
+
+     10 yes   yes   N/A     exists   nothing  remove path from index
+     11 no    yes   N/A     exists   nothing  fail
+     12 yes   no    N/A     exists   nothing  fail
+     13 no    no    N/A     exists   nothing  fail
+
+	clean (H==M)
+       ------
+     14 yes                 exists   exists   keep index
+     15 no                  exists   exists   keep index
+
+        clean I==H  I==M (H!=M)
+       ------------------
+     16 yes   no    no      exists   exists   fail
+     17 no    no    no      exists   exists   fail
+     18 yes   no    yes     exists   exists   keep index
+     19 no    no    yes     exists   exists   keep index
+     20 yes   yes   no      exists   exists   use M
+     21 no    yes   no      exists   exists   fail
+....
+
+In all "keep index" cases, the index entry stays as in the
+original index file.  If the entry is not up to date,
+'git read-tree' keeps the copy in the work tree intact when
+operating under the -u flag.
+
+When this form of 'git read-tree' returns successfully, you can
+see which of the "local changes" that you made were carried forward by running
+`git diff-index --cached $M`.  Note that this does not
+necessarily match what `git diff-index --cached $H` would have
+produced before such a two tree merge.  This is because of cases
+18 and 19 --- if you already had the changes in $M (e.g. maybe
+you picked it up via e-mail in a patch form), `git diff-index
+--cached $H` would have told you about the change before this
+merge, but it would not show in `git diff-index --cached $M`
+output after the two-tree merge.
+
+Case 3 is slightly tricky and needs explanation.  The result from this
+rule logically should be to remove the path if the user staged the removal
+of the path and then switching to a new branch.  That however will prevent
+the initial checkout from happening, so the rule is modified to use M (new
+tree) only when the content of the index is empty.  Otherwise the removal
+of the path is kept as long as $H and $M are the same.
+
+3-Way Merge
+~~~~~~~~~~~
+Each "index" entry has two bits worth of "stage" state. stage 0 is the
+normal one, and is the only one you'd see in any kind of normal use.
+
+However, when you do 'git read-tree' with three trees, the "stage"
+starts out at 1.
+
+This means that you can do
+
+----------------
+$ git read-tree -m <tree1> <tree2> <tree3>
+----------------
+
+and you will end up with an index with all of the <tree1> entries in
+"stage1", all of the <tree2> entries in "stage2" and all of the
+<tree3> entries in "stage3".  When performing a merge of another
+branch into the current branch, we use the common ancestor tree
+as <tree1>, the current branch head as <tree2>, and the other
+branch head as <tree3>.
+
+Furthermore, 'git read-tree' has special-case logic that says: if you see
+a file that matches in all respects in the following states, it
+"collapses" back to "stage0":
+
+   - stage 2 and 3 are the same; take one or the other (it makes no
+     difference - the same work has been done on our branch in
+     stage 2 and their branch in stage 3)
+
+   - stage 1 and stage 2 are the same and stage 3 is different; take
+     stage 3 (our branch in stage 2 did not do anything since the
+     ancestor in stage 1 while their branch in stage 3 worked on
+     it)
+
+   - stage 1 and stage 3 are the same and stage 2 is different take
+     stage 2 (we did something while they did nothing)
+
+The 'git write-tree' command refuses to write a nonsensical tree, and it
+will complain about unmerged entries if it sees a single entry that is not
+stage 0.
+
+OK, this all sounds like a collection of totally nonsensical rules,
+but it's actually exactly what you want in order to do a fast
+merge. The different stages represent the "result tree" (stage 0, aka
+"merged"), the original tree (stage 1, aka "orig"), and the two trees
+you are trying to merge (stage 2 and 3 respectively).
+
+The order of stages 1, 2 and 3 (hence the order of three
+<tree-ish> command-line arguments) are significant when you
+start a 3-way merge with an index file that is already
+populated.  Here is an outline of how the algorithm works:
+
+- if a file exists in identical format in all three trees, it will
+  automatically collapse to "merged" state by 'git read-tree'.
+
+- a file that has _any_ difference what-so-ever in the three trees
+  will stay as separate entries in the index. It's up to "porcelain
+  policy" to determine how to remove the non-0 stages, and insert a
+  merged version.
+
+- the index file saves and restores with all this information, so you
+  can merge things incrementally, but as long as it has entries in
+  stages 1/2/3 (i.e., "unmerged entries") you can't write the result. So
+  now the merge algorithm ends up being really simple:
+
+  * you walk the index in order, and ignore all entries of stage 0,
+    since they've already been done.
+
+  * if you find a "stage1", but no matching "stage2" or "stage3", you
+    know it's been removed from both trees (it only existed in the
+    original tree), and you remove that entry.
+
+  * if you find a matching "stage2" and "stage3" tree, you remove one
+    of them, and turn the other into a "stage0" entry. Remove any
+    matching "stage1" entry if it exists too.  .. all the normal
+    trivial rules ..
+
+You would normally use 'git merge-index' with supplied
+'git merge-one-file' to do this last step.  The script updates
+the files in the working tree as it merges each path and at the
+end of a successful merge.
+
+When you start a 3-way merge with an index file that is already
+populated, it is assumed that it represents the state of the
+files in your work tree, and you can even have files with
+changes unrecorded in the index file.  It is further assumed
+that this state is "derived" from the stage 2 tree.  The 3-way
+merge refuses to run if it finds an entry in the original index
+file that does not match stage 2.
+
+This is done to prevent you from losing your work-in-progress
+changes, and mixing your random changes in an unrelated merge
+commit.  To illustrate, suppose you start from what has been
+committed last to your repository:
+
+----------------
+$ JC=`git rev-parse --verify "HEAD^0"`
+$ git checkout-index -f -u -a $JC
+----------------
+
+You do random edits, without running 'git update-index'.  And then
+you notice that the tip of your "upstream" tree has advanced
+since you pulled from him:
+
+----------------
+$ git fetch git://.... linus
+$ LT=`git rev-parse FETCH_HEAD`
+----------------
+
+Your work tree is still based on your HEAD ($JC), but you have
+some edits since.  Three-way merge makes sure that you have not
+added or modified index entries since $JC, and if you haven't,
+then does the right thing.  So with the following sequence:
+
+----------------
+$ git read-tree -m -u `git merge-base $JC $LT` $JC $LT
+$ git merge-index git-merge-one-file -a
+$ echo "Merge with Linus" | \
+  git commit-tree `git write-tree` -p $JC -p $LT
+----------------
+
+what you would commit is a pure merge between $JC and $LT without
+your work-in-progress changes, and your work tree would be
+updated to the result of the merge.
+
+However, if you have local changes in the working tree that
+would be overwritten by this merge, 'git read-tree' will refuse
+to run to prevent your changes from being lost.
+
+In other words, there is no need to worry about what exists only
+in the working tree.  When you have local changes in a part of
+the project that is not involved in the merge, your changes do
+not interfere with the merge, and are kept intact.  When they
+*do* interfere, the merge does not even start ('git read-tree'
+complains loudly and fails without modifying anything).  In such
+a case, you can simply continue doing what you were in the
+middle of doing, and when your working tree is ready (i.e. you
+have finished your work-in-progress), attempt the merge again.
+
+
+SPARSE CHECKOUT
+---------------
+
+"Sparse checkout" allows populating the working directory sparsely.
+It uses the skip-worktree bit (see linkgit:git-update-index[1]) to tell
+Git whether a file in the working directory is worth looking at.
+
+'git read-tree' and other merge-based commands ('git merge', 'git
+checkout'...) can help maintaining the skip-worktree bitmap and working
+directory update. `$GIT_DIR/info/sparse-checkout` is used to
+define the skip-worktree reference bitmap. When 'git read-tree' needs
+to update the working directory, it resets the skip-worktree bit in the index
+based on this file, which uses the same syntax as .gitignore files.
+If an entry matches a pattern in this file, skip-worktree will not be
+set on that entry. Otherwise, skip-worktree will be set.
+
+Then it compares the new skip-worktree value with the previous one. If
+skip-worktree turns from set to unset, it will add the corresponding
+file back. If it turns from unset to set, that file will be removed.
+
+While `$GIT_DIR/info/sparse-checkout` is usually used to specify what
+files are in, you can also specify what files are _not_ in, using
+negate patterns. For example, to remove the file `unwanted`:
+
+----------------
+/*
+!unwanted
+----------------
+
+Another tricky thing is fully repopulating the working directory when you
+no longer want sparse checkout. You cannot just disable "sparse
+checkout" because skip-worktree bits are still in the index and your working
+directory is still sparsely populated. You should re-populate the working
+directory with the `$GIT_DIR/info/sparse-checkout` file content as
+follows:
+
+----------------
+/*
+----------------
+
+Then you can disable sparse checkout. Sparse checkout support in 'git
+read-tree' and similar commands is disabled by default. You need to
+turn `core.sparseCheckout` on in order to have sparse checkout
+support.
+
+
+SEE ALSO
+--------
+linkgit:git-write-tree[1]; linkgit:git-ls-files[1];
+linkgit:gitignore[5]
+
+GIT
+---
+Part of the linkgit:git[1] suite