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, 57 insertions, 2 deletions

diff --git a/t/t1501-worktree.sh b/t/t1501-worktree.sh
index 2ee88d8a06..8244b3a86f 100755
--- a/t/t1501-worktree.sh
+++ b/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