wt-status: implement opportunisitc index update correctly

The require_clean_work_tree() function calls hold_locked_index()
with die_on_error=0 to signal that it is OK if it fails to obtain
the lock, but unconditionally calls update_index_if_able(), which
will try to write into fd=-1.

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

wt-status.c

@@ -2258,11 +2258,12 @@ int has_uncommitted_changes(int ignore_submodules)
 int require_clean_work_tree(const char *action, const char *hint, int ignore_submodules, int gently)
 {
 	struct lock_file *lock_file = xcalloc(1, sizeof(*lock_file));
-	int err = 0;
+	int err = 0, fd;
 
-	hold_locked_index(lock_file, 0);
+	fd = hold_locked_index(lock_file, 0);
 	refresh_cache(REFRESH_QUIET);
-	update_index_if_able(&the_index, lock_file);
+	if (0 <= fd)
+		update_index_if_able(&the_index, lock_file);
 	rollback_lock_file(lock_file);
 
 	if (has_unstaged_changes(ignore_submodules)) {