largefile: use `self` in repo method instead of `repo`
Most method added (or overwritten) to repo by largefile works on `repo`
instead of `self`. This currently works without trouble because `self` and
`repo` are likely the same. However this is semantically dubious and this may
cause issue for filtering. `self` may be proxy object different from the `repo`
one.
This changeset fix that and use `self` when applicable.