builtin/stash: fix `--keep-index --include-untracked` with empty HEAD

It was reported that creating a stash with `--keep-index
--include-untracked` causes an error when HEAD points to a commit whose
tree is empty:

    $ git stash push --keep-index --include-untracked
    error: pathspec ':/' did not match any file(s) known to git

This error comes from `git checkout --no-overlay $i_tree -- :/`, which
we execute to reset the working tree to the state in our index. As the
tree generated from the index is empty in our case, ':/' does not match
any files and thus causes git-checkout(1) to error out.

Fix the issue by skipping the checkout when the index tree is empty. As
explained in the in-code comment, this should be the correct thing to do
as there is nothing that we'd have to reset in the first place.

Reported-by: Piotr Siupa <piotrsiupa@gmail.com>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 15 insertions, 0 deletions

diff --git a/t/t3903-stash.sh b/t/t3903-stash.sh
index 376cc8f4ab..38906785e2 100755
--- a/t/t3903-stash.sh
+++ b/t/t3903-stash.sh
@@ -1384,6 +1384,21 @@ test_expect_success 'stash --keep-index with file deleted in index does not resu
 	test_path_is_missing to-remove
 '
 
+test_expect_success 'stash --keep-index --include-untracked with empty tree' '
+	test_when_finished "rm -rf empty" &&
+	git init empty &&
+	(
+		cd empty &&
+		git commit --allow-empty --message "empty" &&
+		echo content >file &&
+		git stash push --keep-index --include-untracked &&
+		test_path_is_missing file &&
+		git stash pop &&
+		echo content >expect &&
+		test_cmp expect file
+	)
+'
+
 test_expect_success 'stash apply should succeed with unmodified file' '
 	echo base >file &&
 	git add file &&