Convert resolve_ref+xstrdup to new resolve_refdup function

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/merge.c b/builtin/merge.c
index a1c8534..a3cd5e8 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -1096,6 +1096,7 @@
 	struct commit_list *common = NULL;
 	const char *best_strategy = NULL, *wt_strategy = NULL;
 	struct commit_list **remotes = &remoteheads;
+	void *branch_to_free;
 
 	if (argc == 2 && !strcmp(argv[1], "-h"))
 		usage_with_options(builtin_merge_usage, builtin_merge_options);
@@ -1104,12 +1105,9 @@
 	 * Check if we are _not_ on a detached HEAD, i.e. if there is a
 	 * current branch.
 	 */
-	branch = resolve_ref("HEAD", head_sha1, 0, &flag);
-	if (branch) {
-		if (!prefixcmp(branch, "refs/heads/"))
-			branch += 11;
-		branch = xstrdup(branch);
-	}
+	branch = branch_to_free = resolve_refdup("HEAD", head_sha1, 0, &flag);
+	if (branch && !prefixcmp(branch, "refs/heads/"))
+		branch += 11;
 	if (!branch || is_null_sha1(head_sha1))
 		head_commit = NULL;
 	else
@@ -1520,6 +1518,6 @@
 		ret = suggest_conflicts(option_renormalize);
 
 done:
-	free((char *)branch);
+	free(branch_to_free);
 	return ret;
 }