merge: narrow scope of merge_names

In order to pass the list of parents to fmt_merge_msg(), cmd_merge()
uses this strbuf to create something that look like FETCH_HEAD that
describes commits that are being merged.  This is necessary only
when we are creating the merge commit message ourselves, but was
done unconditionally.

Move the variable and the logic to populate it to confine them in a
block that needs them.

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

builtin/merge.c

@@ -1236,8 +1236,6 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		argc -= 2;
 		remoteheads = collect_parents(head_commit, &head_subsumed, argc, argv);
 	} else {
-		struct strbuf merge_names = STRBUF_INIT;
-
 		/* We are invoked directly as the first-class UI. */
 		head_arg = "HEAD";
 
@@ -1247,11 +1245,14 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		 * to the given message.
 		 */
 		remoteheads = collect_parents(head_commit, &head_subsumed, argc, argv);
-		for (p = remoteheads; p; p = p->next)
-			merge_name(merge_remote_util(p->item)->name, &merge_names);
 
 		if (!have_message || shortlog_len) {
+			struct strbuf merge_names = STRBUF_INIT;
 			struct fmt_merge_msg_opts opts;
+
+			for (p = remoteheads; p; p = p->next)
+				merge_name(merge_remote_util(p->item)->name, &merge_names);
+
 			memset(&opts, 0, sizeof(opts));
 			opts.add_title = !have_message;
 			opts.shortlog_len = shortlog_len;
@@ -1260,6 +1261,8 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 			fmt_merge_msg(&merge_names, &merge_msg, &opts);
 			if (merge_msg.len)
 				strbuf_setlen(&merge_msg, merge_msg.len - 1);
+
+			strbuf_release(&merge_names);
 		}
 	}