git-diff: Add --staged as a synonym for --cached.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-diff.txt

@@ -33,6 +33,7 @@ forced by --no-index.
 	commit relative to the named <commit>.  Typically you
 	would want comparison with the latest commit, so if you
 	do not give <commit>, it defaults to HEAD.
+	--staged is a synonym of --cached.
 
 'git diff' [--options] <commit> [--] [<path>...]::
 

builtin-diff.c

@@ -118,7 +118,7 @@ static int builtin_diff_index(struct rev_info *revs,
 	int cached = 0;
 	while (1 < argc) {
 		const char *arg = argv[1];
-		if (!strcmp(arg, "--cached"))
+		if (!strcmp(arg, "--cached") || !strcmp(arg, "--staged"))
 			cached = 1;
 		else
 			usage(builtin_diff_usage);
@@ -319,7 +319,8 @@ int cmd_diff(int argc, const char **argv, const char *prefix)
 			const char *arg = argv[i];
 			if (!strcmp(arg, "--"))
 				break;
-			else if (!strcmp(arg, "--cached")) {
+			else if (!strcmp(arg, "--cached") ||
+				 !strcmp(arg, "--staged")) {
 				add_head_to_pending(&rev);
 				if (!rev.pending.nr)
 					die("No HEAD commit to compare with (yet)");