Teach mailinfo %< as an alternative scissors mark

Handle perforations found “in the wild” more robustly by recognizing
“%<” as an alternative scissors mark.

This feature is only meant to support old habits.  Discourage new use
of the percent-based version by only documenting the 8< symbol so new
users’ perforations can still be recognized by old versions of Git.
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/mailinfo.c

@@ -746,7 +746,8 @@ static int is_scissors_line(const struct strbuf *line)
 			continue;
 		}
 		if (i + 1 < len &&
-		    (!memcmp(buf + i, ">8", 2) || !memcmp(buf + i, "8<", 2))) {
+		    (!memcmp(buf + i, ">8", 2) || !memcmp(buf + i, "8<", 2) ||
+		     !memcmp(buf + i, ">%", 2) || !memcmp(buf + i, "%<", 2))) {
 			in_perforation = 1;
 			perforation += 2;
 			scissors += 2;