reflog: move the reflog implementation into refdb_fs

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.
reflog: add a convenience append function

Provide a function that reads a reflog, performs an append and writes back to the
backend in one call.
reflog: bring _append and _drop back to the frontend

These functions act purely on the reflog data structure.
Merge pull request #1802 from libgit2/cmn/reflog-backend

Make reflog part of refdb