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.
