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, 3 insertions, 0 deletions

diff --git a/builtin-diff.c b/builtin-diff.c
index 7ffea9750..037c3039a 100644
--- a/builtin-diff.c
+++ b/builtin-diff.c
@@ -122,6 +122,8 @@ static int builtin_diff_index(struct rev_info *revs,
 			usage(builtin_diff_usage);
 		argv++; argc--;
 	}
+	if (!cached)
+		setup_work_tree();
 	/*
 	 * Make sure there is one revision (i.e. pending object),
 	 * and there is no revision filtering parameters.
@@ -225,6 +227,7 @@ static int builtin_diff_files(struct rev_info *revs, int argc, const char **argv
 	    (revs->diffopt.output_format & DIFF_FORMAT_PATCH))
 		revs->combine_merges = revs->dense_combined_merges = 1;
 
+	setup_work_tree();
 	if (read_cache() < 0) {
 		perror("read_cache");
 		return -1;