fmt-merge-msg: avoid leaking strbuf in shortlog()

Use string_list_append_nodup() instead of string_list_append() to hand
over ownership of a detached strbuf and thus avoid leaking its memory.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fmt-merge-msg.c

@@ -376,7 +376,8 @@ static void shortlog(const char *name,
 			string_list_append(&subjects,
 					   oid_to_hex(&commit->object.oid));
 		else
-			string_list_append(&subjects, strbuf_detach(&sb, NULL));
+			string_list_append_nodup(&subjects,
+						 strbuf_detach(&sb, NULL));
 	}
 
 	if (opts->credit_people)