revision: change "--diff-merges" option to require parameter

--diff-merges=off is the only accepted form for now, a synonym for
--no-diff-merges.

This patch is a preparation for adding more values, as well as supporting
--diff-merges=<parent>, where <parent> is single parent number to output diff
against.

Signed-off-by: Sergey Organov <sorganov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

revision.c

@@ -2323,8 +2323,15 @@ static int handle_revision_opt(struct rev_info *revs, int argc, const char **arg
 		revs->diff = 1;
 		revs->diffopt.flags.recursive = 1;
 		revs->diffopt.flags.tree_in_recursive = 1;
-	} else if (!strcmp(arg, "-m") || !strcmp(arg, "--diff-merges")) {
+	} else if (!strcmp(arg, "-m")) {
 		revs->ignore_merges = 0;
+	} else if ((argcount = parse_long_opt("diff-merges", argv, &optarg))) {
+		if (!strcmp(optarg, "off")) {
+			revs->ignore_merges = 1;
+		} else {
+			die(_("unknown value for --diff-merges: %s"), optarg);
+		}
+		return argcount;
 	} else if (!strcmp(arg, "--no-diff-merges")) {
 		revs->ignore_merges = 1;
 	} else if (!strcmp(arg, "-c")) {