grep: fix worktree setup

Unless used with --cached or grepping on a tree, "git grep" will
search on working directory, so set up worktree properly

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-grep.c

@@ -783,8 +783,11 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 		paths[1] = NULL;
 	}
 
-	if (!list.nr)
+	if (!list.nr) {
+		if (!cached)
+			setup_work_tree();
 		return !grep_cache(&opt, paths, cached);
+	}
 
 	if (cached)
 		die("both --cached and trees are given.");

t/t1501-worktree.sh

@@ -169,4 +169,9 @@ test_expect_success 'git diff' '
 	test_cmp diff-FILES.expected result
 '
 
+test_expect_success 'git grep' '
+	(cd repo.git/work/sub &&
+	GIT_DIR=../.. GIT_WORK_TREE=.. git grep -l changed | grep -q dir/tracked)
+'
+
 test_done