References and their logs are logically coupled, let's make it so in
the code by moving the fs-based reflog implementation to live next to
the fs-based refs one.
As part of the change, make the function take names rather than
references, as only the names are relevant when looking up and
handling reflogs.
Nobody should ever be using anything other than ALL at this level, so
remove the option altogether.
As part of this, git_reference_foreach_glob is now implemented in the
frontend using an iterator. Backends will later regain the ability of
doing the glob filtering in the backend.
