clean: demonstrate a bug with pathspecs

b9670c1f5e (dir: fix checks on common prefix directory, 2019-12-19) modified the way pathspecs are handled when handling a directory during "git clean -f <path>". While this improved the behavior for known test breakages, it also regressed in how the clean command handles cleaning a specified file. Add a test case that demonstrates this behavior. This test passes before b9670c1f5e then fails after. Helped-by: Kevin Willford <Kevin.Willford@microsoft.com> Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Reviewed-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>