Add unit tests to confirm ignore directory pattern matches and
to confirm that ignore and attribute files are loaded properly
into the attribute file cache.
When building an attr path object, the code that checks if the
file is a directory was evaluating the file as a relative path
to the current working directory, instead of using the repo root.
This lead to inconsistent behavior.
git_refspec_transform_r assumed that the reference name passed would
be only a branch or tag name. This is not the case, and we need to
take into consideration what's in the refspec's source to know how
much of the prefix to ignore.
