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>

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;

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;

git.c

@@ -286,7 +286,7 @@ static void handle_internal_command(int argc, const char **argv)
 		{ "count-objects", cmd_count_objects, RUN_SETUP },
 		{ "describe", cmd_describe, RUN_SETUP },
 		{ "diff", cmd_diff },
-		{ "diff-files", cmd_diff_files, RUN_SETUP },
+		{ "diff-files", cmd_diff_files, RUN_SETUP | NEED_WORK_TREE },
 		{ "diff-index", cmd_diff_index, RUN_SETUP },
 		{ "diff-tree", cmd_diff_tree, RUN_SETUP },
 		{ "fast-export", cmd_fast_export, RUN_SETUP },

t/t1501-worktree.sh

@@ -28,6 +28,7 @@ test_rev_parse() {
 	[ $# -eq 0 ] && return
 }
 
+EMPTY_TREE=$(git write-tree)
 mkdir -p work/sub/dir || exit 1
 mv .git repo.git || exit 1
 
@@ -106,12 +107,66 @@ test_expect_success 'repo finds its work tree from work tree, too' '
 '
 
 test_expect_success '_gently() groks relative GIT_DIR & GIT_WORK_TREE' '
-	cd repo.git/work/sub/dir &&
+	(cd repo.git/work/sub/dir &&
 	GIT_DIR=../../.. GIT_WORK_TREE=../.. GIT_PAGER= \
 		git diff --exit-code tracked &&
 	echo changed > tracked &&
 	! GIT_DIR=../../.. GIT_WORK_TREE=../.. GIT_PAGER= \
-		git diff --exit-code tracked
+		git diff --exit-code tracked)
+'
+cat > diff-index-cached.expected <<\EOF
+:000000 100644 0000000000000000000000000000000000000000 e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 A	sub/dir/tracked
+EOF
+cat > diff-index.expected <<\EOF
+:000000 100644 0000000000000000000000000000000000000000 0000000000000000000000000000000000000000 A	sub/dir/tracked
+EOF
+
+
+test_expect_success 'git diff-index' '
+	GIT_DIR=repo.git GIT_WORK_TREE=repo.git/work git diff-index $EMPTY_TREE > result &&
+	test_cmp diff-index.expected result &&
+	GIT_DIR=repo.git git diff-index --cached $EMPTY_TREE > result &&
+	test_cmp diff-index-cached.expected result
+'
+cat >diff-files.expected <<\EOF
+:100644 100644 e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 0000000000000000000000000000000000000000 M	sub/dir/tracked
+EOF
+
+test_expect_success 'git diff-files' '
+	GIT_DIR=repo.git GIT_WORK_TREE=repo.git/work git diff-files > result &&
+	test_cmp diff-files.expected result
+'
+
+cat >diff-TREE.expected <<\EOF
+diff --git a/sub/dir/tracked b/sub/dir/tracked
+new file mode 100644
+index 0000000..5ea2ed4
+--- /dev/null
++++ b/sub/dir/tracked
+@@ -0,0 +1 @@
++changed
+EOF
+cat >diff-TREE-cached.expected <<\EOF
+diff --git a/sub/dir/tracked b/sub/dir/tracked
+new file mode 100644
+index 0000000..e69de29
+EOF
+cat >diff-FILES.expected <<\EOF
+diff --git a/sub/dir/tracked b/sub/dir/tracked
+index e69de29..5ea2ed4 100644
+--- a/sub/dir/tracked
++++ b/sub/dir/tracked
+@@ -0,0 +1 @@
++changed
+EOF
+
+test_expect_success 'git diff' '
+	GIT_DIR=repo.git GIT_WORK_TREE=repo.git/work git diff $EMPTY_TREE > result &&
+	test_cmp diff-TREE.expected result &&
+	GIT_DIR=repo.git git diff --cached $EMPTY_TREE > result &&
+	test_cmp diff-TREE-cached.expected result &&
+	GIT_DIR=repo.git GIT_WORK_TREE=repo.git/work git diff > result &&
+	test_cmp diff-FILES.expected result
 '
 
 test_done