git-p4: add failing test for "don't exclude other files with same prefix"

git-p4: add failing test for "don't exclude other files with same prefix"

In preparation for a fix, add a failing test case to test that
git-p4 doesn't exclude files with the same prefix unintentionally
when exclude paths are specified without a trailing /.
I.e., don't exclude "//depot/file_dont_exclude" if run with "-//depot/file".
or don't exclude "//depot/discard_file_not" if run with "-//depot/discard_file".

Signed-off-by: Andrey Mazo <amazo@checkvideo.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>