This adds a BARE option to git_repository_open_ext which allows
a fast open path that still knows how to read gitlinks and to
search for the actual .git directory from a subdirectory.

`git_repository_open_bare` is still simpler and faster, but having
a gitlink aware fast open is very useful for submodules where we
want to quickly be able to peek at the HEAD and index data without
doing any other meaningful repo operations.