diff-merges: add '--diff-merges=1' as synonym for 'first-parent'

As we now have --diff-merges={m|c|cc}, add --diff-merges=1 as synonym
for --diff-merges=first-parent, to have shorter mnemonics for it as
well.

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

diff-merges.c

@@ -58,7 +58,7 @@ static void set_diff_merges(struct rev_info *revs, const char *optarg)
 		return;
 	}
 
-	if (!strcmp(optarg, "first-parent"))
+	if (!strcmp(optarg, "1") || !strcmp(optarg, "first-parent"))
 		set_first_parent(revs);
 	else if (!strcmp(optarg, "m") || !strcmp(optarg, "separate"))
 		set_separate(revs);