revision.c: explain what tree_difference does

diff --git a/Documentation/git-diff-tree.txt b/Documentation/git-diff-tree.txt
index 9a2947e27dc2476a78bad93957cca18a06ab037b..5d6e9dc751aef6e30582b399ae09f81c043e6de1 100644 (file)
--- a/Documentation/git-diff-tree.txt
+++ b/Documentation/git-diff-tree.txt
@@ -8,7 +8,10 @@ git-diff-tree - Compares the content and mode of blobs found via two tree object
 
 SYNOPSIS
 --------
-'git-diff-tree' [--stdin] [-m] [-s] [-v] [--no-commit-id] [--pretty] [-t] [-r] [--root] [<common diff options>] <tree-ish> [<tree-ish>] [<path>...]
+[verse]
+'git-diff-tree' [--stdin] [-m] [-s] [-v] [--no-commit-id] [--pretty]
+             [-t] [-r] [-c | --cc] [--root] [<common diff options>]
+             <tree-ish> [<tree-ish>] [<path>...]
 
 DESCRIPTION
 -----------
@@ -29,7 +32,7 @@ include::diff-options.txt[]
 <path>...::
        If provided, the results are limited to a subset of files
        matching one of these prefix strings.
-       ie file matches `/^<pattern1>|<pattern2>|.../`
+       i.e., file matches `/^<pattern1>|<pattern2>|.../`
        Note that this parameter does not provide any wildcard or regexp
        features.
 
@@ -51,33 +54,55 @@ include::diff-options.txt[]
 +
 When a single commit is given on one line of such input, it compares
 the commit with its parents.  The following flags further affects its
-behaviour.  This does not apply to the case where two <tree-ish>
+behavior.  This does not apply to the case where two <tree-ish>
 separated with a single space are given.
 
 -m::
        By default, "git-diff-tree --stdin" does not show
        differences for merge commits.  With this flag, it shows
-       differences to that commit from all of its parents.
+       differences to that commit from all of its parents. See
+       also '-c'.
 
 -s::
        By default, "git-diff-tree --stdin" shows differences,
        either in machine-readable form (without '-p') or in patch
-       form (with '-p').  This output can be supressed.  It is
+       form (with '-p').  This output can be suppressed.  It is
        only useful with '-v' flag.
 
 -v::
        This flag causes "git-diff-tree --stdin" to also show
        the commit message before the differences.
 
---pretty[=(raw|medium|short)]::
-       This is used to control "pretty printing" format of the
-       commit message.  Without "=<style>", it defaults to
-       medium.
+include::pretty-formats.txt[]
 
 --no-commit-id::
        git-diff-tree outputs a line with the commit ID when
        applicable.  This flag suppressed the commit ID output.
 
+-c::
+       This flag changes the way a merge commit is displayed
+       (which means it is useful only when the command is given
+       one <tree-ish>, or '--stdin').  It shows the differences
+       from each of the parents to the merge result simultaneously
+       instead of showing pairwise diff between a parent and the
+       result one at a time (which is what the '-m' option does).
+       Furthermore, it lists only files which were modified
+       from all parents.
+
+--cc::
+       This flag changes the way a merge commit patch is displayed,
+       in a similar way to the '-c' option. It implies the '-c'
+       and '-p' options and further compresses the patch output
+       by omitting hunks that show differences from only one
+       parent, or show the same change from all but one parent
+       for an Octopus merge.  When this optimization makes all
+       hunks disappear, the commit itself and the commit log
+       message is not shown, just like in any other "empty diff" case.
+
+--always::
+       Show the commit itself and the commit log message even
+       if the diff itself is empty.
+
 
 Limiting Output
 ---------------