git-diff: fix missing --merge-base docs

When `git diff --merge-base` was introduced at around Git 2.30, the
documentation included a few errors.

In the example given for `git diff --cached --merge-base`, the
`--cached` flag was omitted for the `--merge-base` example. Add the
missing flag.

In the `git diff <commit>` case, we failed to mention that
`--merge-base` is an available option. Give the usage of `--merge-base`
as an option there.

Finally, there are two errors in the usage of `git diff`. Firstly, we do
not mention `--merge-base` in the `git diff --cached` case. Mention it
so that it's consistent with the documentation. Secondly, we put the
`[--merge-base]` in between `<commit>` and `[<commit>...]`. Move the
`[--merge-base]` so that it's beside `[<options>]` which is a more
logical grouping.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-diff.txt

@@ -51,16 +51,20 @@ files on disk.
 	--staged is a synonym of --cached.
 +
 If --merge-base is given, instead of using <commit>, use the merge base
-of <commit> and HEAD.  `git diff --merge-base A` is equivalent to
-`git diff $(git merge-base A HEAD)`.
+of <commit> and HEAD.  `git diff --cached --merge-base A` is equivalent to
+`git diff --cached $(git merge-base A HEAD)`.
 
-'git diff' [<options>] <commit> [--] [<path>...]::
+'git diff' [<options>] [--merge-base] <commit> [--] [<path>...]::
 
 	This form is to view the changes you have in your
 	working tree relative to the named <commit>.  You can
 	use HEAD to compare it with the latest commit, or a
 	branch name to compare with the tip of a different
 	branch.
++
+If --merge-base is given, instead of using <commit>, use the merge base
+of <commit> and HEAD.  `git diff --merge-base A` is equivalent to
+`git diff $(git merge-base A HEAD)`.
 
 'git diff' [<options>] [--merge-base] <commit> <commit> [--] [<path>...]::
 

builtin/diff.c

@@ -25,8 +25,8 @@
 
 static const char builtin_diff_usage[] =
 "git diff [<options>] [<commit>] [--] [<path>...]\n"
-"   or: git diff [<options>] --cached [<commit>] [--] [<path>...]\n"
-"   or: git diff [<options>] <commit> [--merge-base] [<commit>...] <commit> [--] [<path>...]\n"
+"   or: git diff [<options>] --cached [--merge-base] [<commit>] [--] [<path>...]\n"
+"   or: git diff [<options>] [--merge-base] <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"