'git clean -fd' must not delete an untracked directory if it belongs
to a different Git repository or worktree. Unfortunately, if a
'.gitignore' rule in the outer repository happens to match a file in a
nested repository or worktree, then something goes awry and 'git clean
-fd' does delete the content of the nested repository's worktree
except that ignored file, potentially leading to data loss.
Add a test to 't7300-clean.sh' to demonstrate this breakage.
This issue is a regression introduced in
6b1db43109 (clean: teach
clean -d to preserve ignored paths, 2017-05-23).
Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>