dir.c::test_one_path(): work around directory_exists_in_index_icase() breakage

directory_exists_in_index() takes pathname and its length, but its
helper function directory_exists_in_index_icase() reads one byte
beyond the end of the pathname and expects there to be a '/'.

This needs to be fixed, as that one-byte-beyond-the-end location may
not even be readable, possibly by not registering directories to
name hashes with trailing slashes.  In the meantime, update the new
caller added recently to treat_one_path() to make sure that the path
buffer it gives the function is one byte longer than the path it is
asking the function about by appending a slash to it.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>