commit: do not lose mergetag header when not amending

The earlier ed7a42a (commit: teach --amend to carry forward extra headers,
2011-11-08) broke "git merge/pull; edit to fix conflict; git commit"
workflow by forgetting that commit_tree_extended() takes the whole extra
header list.

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

builtin/commit.c

@@ -1484,8 +1484,12 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 		exit(1);
 	}
 
-	if (amend)
+	if (amend) {
 		extra = read_commit_extra_headers(current_head);
+	} else {
+		struct commit_extra_header **tail = &extra;
+		append_merge_tag_headers(parents, &tail);
+	}
 
 	if (commit_tree_extended(sb.buf, active_cache_tree->sha1, parents, sha1,
 				 author_ident.buf, extra)) {