diff*: fix worktree setup

This fixes "git diff", "git diff-files" and "git diff-index" to work
correctly under worktree setup. Because diff* family works in many modes
and not all of them require worktree, Junio made a nice summary
(with a little modification from me):

 * diff-files is about comparing with work tree, so it obviously needs a
  work tree;

 * diff-index also does, except "diff-index --cached" or "diff --cached TREE"

 * no-index is about random files outside git context, so it obviously
   doesn't need any work tree;

 * comparing two (or more) trees doesn't;

 * comparing two blobs doesn't;

 * comparing a blob with a random file doesn't;

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

1 files changed, 2 insertions, 0 deletions

diff --git a/builtin-diff-index.c b/builtin-diff-index.c
index 17d851b29e..04837494fe 100644
--- a/builtin-diff-index.c
+++ b/builtin-diff-index.c
@@ -39,6 +39,8 @@ int cmd_diff_index(int argc, const char **argv, const char *prefix)
 	if (rev.pending.nr != 1 ||
 	    rev.max_count != -1 || rev.min_age != -1 || rev.max_age != -1)
 		usage(diff_cache_usage);
+	if (!cached)
+		setup_work_tree();
 	if (read_cache() < 0) {
 		perror("read_cache");
 		return -1;