This was a convenience method for the refs front-end to do the reflog
writing. This is now done in the backend and it has no more reason for
being.

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

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.

Passing 0 as the index now retrieves the most recent entry instead
of the oldest one.