Documentation: usage for diff combined commits

Document the usage for producing combined commits with "git diff".
This includes updating the synopsis section.

While here, add the three-dot notation to the synopsis.

Make "git diff -h" print the same usage summary as the manual
page synopsis, minus the "A..B" form, which is now discouraged.

Signed-off-by: Chris Torek <chris.torek@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-diff.txt

@@ -11,15 +11,17 @@ SYNOPSIS
 [verse]
 'git diff' [<options>] [<commit>] [--] [<path>...]
 'git diff' [<options>] --cached [<commit>] [--] [<path>...]
-'git diff' [<options>] <commit> <commit> [--] [<path>...]
+'git diff' [<options>] <commit> [<commit>...] <commit> [--] [<path>...]
+'git diff' [<options>] <commit>...<commit> [--] [<path>...]
 'git diff' [<options>] <blob> <blob>
 'git diff' [<options>] --no-index [--] <path> <path>
 
 DESCRIPTION
 -----------
 Show changes between the working tree and the index or a tree, changes
-between the index and a tree, changes between two trees, changes between
-two blob objects, or changes between two files on disk.
+between the index and a tree, changes between two trees, changes resulting
+from a merge, changes between two blob objects, or changes between two
+files on disk.
 
 'git diff' [<options>] [--] [<path>...]::
 
@@ -67,6 +69,15 @@ two blob objects, or changes between two files on disk.
 	one side is omitted, it will have the same effect as
 	using HEAD instead.
 
+'git diff' [<options>] <commit> [<commit>...] <commit> [--] [<path>...]::
+
+	This form is to view the results of a merge commit.  The first
+	listed <commit> must be the merge itself; the remaining two or
+	more commits should be its parents.  A convenient way to produce
+	the desired set of revisions is to use the {caret}@ suffix.
+	For instance, if `master` names a merge commit, `git diff master
+	master^@` gives the same combined diff as `git show master`.
+
 'git diff' [<options>] <commit>\...<commit> [--] [<path>...]::
 
 	This form is to view the changes on the branch containing
@@ -196,7 +207,8 @@ linkgit:git-difftool[1],
 linkgit:git-log[1],
 linkgit:gitdiffcore[7],
 linkgit:git-format-patch[1],
-linkgit:git-apply[1]
+linkgit:git-apply[1],
+linkgit:git-show[1]
 
 GIT
 ---

builtin/diff.c

@@ -24,7 +24,13 @@
 #define DIFF_NO_INDEX_IMPLICIT 2
 
 static const char builtin_diff_usage[] =
-"git diff [<options>] [<commit> [<commit>]] [--] [<path>...]";
+"git diff [<options>] [<commit>] [--] [<path>...]\n"
+"   or: git diff [<options>] --cached [<commit>] [--] [<path>...]\n"
+"   or: git diff [<options>] <commit> [<commit>...] <commit> [--] [<path>...]\n"
+"   or: git diff [<options>] <commit>...<commit>] [--] [<path>...]\n"
+"   or: git diff [<options>] <blob> <blob>]\n"
+"   or: git diff [<options>] --no-index [--] <path> <path>]\n"
+COMMON_DIFF_OPTIONS_HELP;
 
 static const char *blob_path(struct object_array_entry *entry)
 {