merge-tree: implement real merges

This adds the ability to perform real merges rather than just trivial
merges (meaning handling three way content merges, recursive ancestor
consolidation, renames, proper directory/file conflict handling, and so
forth).  However, unlike `git merge`, the working tree and index are
left alone and no branch is updated.

The only output is:
  - the toplevel resulting tree printed on stdout
  - exit status of 0 (clean), 1 (conflicts present), anything else
    (merge could not be performed; unknown if clean or conflicted)

This output is meant to be used by some higher level script, perhaps in
a sequence of steps like this:

   NEWTREE=$(git merge-tree --write-tree $BRANCH1 $BRANCH2)
   test $? -eq 0 || die "There were conflicts..."
   NEWCOMMIT=$(git commit-tree $NEWTREE -p $BRANCH1 -p $BRANCH2)
   git update-ref $BRANCH1 $NEWCOMMIT

Note that higher level scripts may also want to access the
conflict/warning messages normally output during a merge, or have quick
access to a list of files with conflicts.  That is not available in this
preliminary implementation, but subsequent commits will add that
ability (meaning that NEWTREE would be a lot more than a tree in the
case of conflicts).

This also marks the traditional trivial merge of merge-tree as
deprecated.  The trivial merge not only had limited applicability, the
output format was also difficult to work with (and its format
undocumented), and will generally be less performant than real merges.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 86 insertions, 12 deletions

diff --git a/Documentation/git-merge-tree.txt b/Documentation/git-merge-tree.txt
index 58731c1942..2a9c91328d 100644
--- a/Documentation/git-merge-tree.txt
+++ b/Documentation/git-merge-tree.txt
@@ -3,26 +3,100 @@ git-merge-tree(1)
 
 NAME
 ----
-git-merge-tree - Show three-way merge without touching index
+git-merge-tree - Perform merge without touching index or working tree
 
 
 SYNOPSIS
 --------
 [verse]
-'git merge-tree' <base-tree> <branch1> <branch2>
+'git merge-tree' [--write-tree] <branch1> <branch2>
+'git merge-tree' [--trivial-merge] <base-tree> <branch1> <branch2> (deprecated)
 
+[[NEWMERGE]]
 DESCRIPTION
 -----------
-Reads three tree-ish, and output trivial merge results and
-conflicting stages to the standard output.  This is similar to
-what three-way 'git read-tree -m' does, but instead of storing the
-results in the index, the command outputs the entries to the
-standard output.
-
-This is meant to be used by higher level scripts to compute
-merge results outside of the index, and stuff the results back into the
-index.  For this reason, the output from the command omits
-entries that match the <branch1> tree.
+
+This command has a modern `--write-tree` mode and a deprecated
+`--trivial-merge` mode.  With the exception of the
+<<DEPMERGE,DEPRECATED DESCRIPTION>> section at the end, the rest of
+this documentation describes modern `--write-tree` mode.
+
+Performs a merge, but does not make any new commits and does not read
+from or write to either the working tree or index.
+
+The performed merge will use the same feature as the "real"
+linkgit:git-merge[1], including:
+
+  * three way content merges of individual files
+  * rename detection
+  * proper directory/file conflict handling
+  * recursive ancestor consolidation (i.e. when there is more than one
+    merge base, creating a virtual merge base by merging the merge bases)
+  * etc.
+
+After the merge completes, a new toplevel tree object is created.  See
+`OUTPUT` below for details.
+
+[[OUTPUT]]
+OUTPUT
+------
+
+For either a successful or conflicted merge, the output from
+git-merge-tree is simply one line:
+
+	<OID of toplevel tree>
+
+The printed tree object corresponds to what would be checked out in
+the working tree at the end of `git merge`, and thus may have files
+with conflict markers in them.
+
+EXIT STATUS
+-----------
+
+For a successful, non-conflicted merge, the exit status is 0.  When the
+merge has conflicts, the exit status is 1.  If the merge is not able to
+complete (or start) due to some kind of error, the exit status is
+something other than 0 or 1 (and the output is unspecified).
+
+USAGE NOTES
+-----------
+
+This command is intended as low-level plumbing, similar to
+linkgit:git-hash-object[1], linkgit:git-mktree[1],
+linkgit:git-commit-tree[1], linkgit:git-write-tree[1],
+linkgit:git-update-ref[1], and linkgit:git-mktag[1].  Thus, it can be
+used as a part of a series of steps such as:
+
+       NEWTREE=$(git merge-tree --write-tree $BRANCH1 $BRANCH2)
+       test $? -eq 0 || die "There were conflicts..."
+       NEWCOMMIT=$(git commit-tree $NEWTREE -p $BRANCH1 -p $BRANCH2)
+       git update-ref $BRANCH1 $NEWCOMMIT
+
+[[DEPMERGE]]
+DEPRECATED DESCRIPTION
+----------------------
+
+Per the <<NEWMERGE,DESCRIPTION>> and unlike the rest of this
+documentation, this section describes the deprecated `--trivial-merge`
+mode.
+
+Other than the optional `--trivial-merge`, this mode accepts no
+options.
+
+This mode reads three tree-ish, and outputs trivial merge results and
+conflicting stages to the standard output in a semi-diff format.
+Since this was designed for higher level scripts to consume and merge
+the results back into the index, it omits entries that match
+<branch1>.  The result of this second form is similar to what
+three-way 'git read-tree -m' does, but instead of storing the results
+in the index, the command outputs the entries to the standard output.
+
+This form not only has limited applicability (a trivial merge cannot
+handle content merges of individual files, rename detection, proper
+directory/file conflict handling, etc.), the output format is also
+difficult to work with, and it will generally be less performant than
+the first form even on successful merges (especially if working in
+large repositories).
 
 GIT
 ---