clean: demonstrate a bug with pathspecs

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 <redacted>
Signed-off-by: Derrick Stolee <redacted>
Reviewed-by: Elijah Newren <redacted>
Signed-off-by: Junio C Hamano <redacted>