lock_ref_sha1(): do not sometimes error() and sometimes die().

This cleans up the error path in the function so it does not
die() itself sometimes while signalling an error with NULL some
other times which was inconsistent and confusing.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

refs.c

@@ -561,6 +561,7 @@ static struct ref_lock *lock_ref_sha1_basic(const char *ref, const unsigned char
 	const char *orig_ref = ref;
 	struct ref_lock *lock;
 	struct stat st;
+	int last_errno = 0;
 	int mustexist = (old_sha1 && !is_null_sha1(old_sha1));
 
 	lock = xcalloc(1, sizeof(struct ref_lock));
@@ -574,17 +575,18 @@ static struct ref_lock *lock_ref_sha1_basic(const char *ref, const unsigned char
 		 * to remain.
 		 */
 		ref_file = git_path("%s", orig_ref);
-		if (remove_empty_directories(ref_file))
-			die("there are still refs under '%s'", orig_ref);
+		if (remove_empty_directories(ref_file)) {
+			last_errno = errno;
+			error("there are still refs under '%s'", orig_ref);
+			goto error_return;
+		}
 		ref = resolve_ref(orig_ref, lock->old_sha1, mustexist, NULL);
 	}
 	if (!ref) {
-		int last_errno = errno;
+		last_errno = errno;
 		error("unable to resolve reference %s: %s",
 			orig_ref, strerror(errno));
-		unlock_ref(lock);
-		errno = last_errno;
-		return NULL;
+		goto error_return;
 	}
 	lock->lk = xcalloc(1, sizeof(struct lock_file));
 
@@ -593,11 +595,19 @@ static struct ref_lock *lock_ref_sha1_basic(const char *ref, const unsigned char
 	ref_file = git_path("%s", ref);
 	lock->force_write = lstat(ref_file, &st) && errno == ENOENT;
 
-	if (safe_create_leading_directories(ref_file))
-		die("unable to create directory for %s", ref_file);
+	if (safe_create_leading_directories(ref_file)) {
+		last_errno = errno;
+		error("unable to create directory for %s", ref_file);
+		goto error_return;
+	}
 	lock->lock_fd = hold_lock_file_for_update(lock->lk, ref_file, 1);
 
 	return old_sha1 ? verify_lock(lock, old_sha1, mustexist) : lock;
+
+ error_return:
+	unlock_ref(lock);
+	errno = last_errno;
+	return NULL;
 }
 
 struct ref_lock *lock_ref_sha1(const char *ref, const unsigned char *old_sha1)