mailinfo: off-by-one fix for [PATCH (foobar)] removal from Subject: line

A patch title "[PATCH] 1" was sanitized by the original code by stripping
the "[PATCH]" from the front, but after the conversion to use strbuf this
behaviour was broken due to a counting error.

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

builtin-mailinfo.c

@@ -225,10 +225,9 @@ static void cleanup_subject(struct strbuf *subject)
 			continue;
 		case '[':
 			if ((pos = strchr(subject->buf, ']'))) {
-				remove = pos - subject->buf + 1;
-				/* Don't remove too much. */
-				if (remove <= (subject->len - remove + 1) * 2) {
-					strbuf_remove(subject, 0, remove);
+				remove = pos - subject->buf;
+				if (remove <= (subject->len - remove) * 2) {
+					strbuf_remove(subject, 0, remove + 1);
 					continue;
 				}
 			} else