hold_lock_file_for_append(): restore errno before returning

Callers who don't pass LOCK_DIE_ON_ERROR might want to examine errno
to see what went wrong, so restore errno before returning.

In fact this function only has one caller, add_to_alternates_file(),
and it *does* use LOCK_DIE_ON_ERROR, but, you know, think of future
generations.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

lockfile.c

@@ -243,15 +243,22 @@ int hold_lock_file_for_append(struct lock_file *lk, const char *path, int flags)
 	orig_fd = open(path, O_RDONLY);
 	if (orig_fd < 0) {
 		if (errno != ENOENT) {
+			int save_errno = errno;
+
 			if (flags & LOCK_DIE_ON_ERROR)
 				die("cannot open '%s' for copying", path);
 			rollback_lock_file(lk);
-			return error("cannot open '%s' for copying", path);
+			error("cannot open '%s' for copying", path);
+			errno = save_errno;
+			return -1;
 		}
 	} else if (copy_fd(orig_fd, fd)) {
+		int save_errno = errno;
+
 		if (flags & LOCK_DIE_ON_ERROR)
 			exit(128);
 		rollback_lock_file(lk);
+		errno = save_errno;
 		return -1;
 	}
 	return fd;