[PATCH] git-commit-script fix for degenerated merge

If merging results in an unchanged tree, git-commit-script should not
complain that there's nothing to commit.

Also, add "[--all]" to usage().

[jc: usually there is no reason to record an unchanging merge,
but this code path is triggered only when there is a nontrivial
merge that needed to be resolved by hand, and we should be able
to record the fact that these two tree heads are dealt with as a
regular two-parent commit in order to help later merges.]

Signed-off-by: Junio C Hamano <junkio@cox.net>

git-commit-script

@@ -6,7 +6,7 @@
 . git-sh-setup-script || die "Not a git archive"
 
 usage () {
-	die 'git commit [-m existing-commit] [<path>...]'
+	die 'git commit [--all] [-m existing-commit] [<path>...]'
 }
 
 files=()
@@ -86,7 +86,7 @@ else
 	fi >.editmsg
 	git-status-script >>.editmsg
 fi
-if [ "$?" != "0" ]
+if [ "$?" != "0" -a ! -f $GIT_DIR/MERGE_HEAD ]
 then
 	cat .editmsg
 	rm .editmsg