share: stop using 'islocal' with repo instance
Having this level of polymorphism of the `islocal` function is weird, and we
already have a way to know if the repo is local from the object itself.
We are about to deprecate passing a non-bytes object to `islocal`, so clean this
up beforehand.
We might want to clean up this function too in the future, however this is
another adventure.