refs: pass refname when invoking reflog entry callback

refs: pass refname when invoking reflog entry callback

With `refs_for_each_reflog_ent()` callers can iterate through all the
reflog entries for a given reference. The callback that is being invoked
for each such entry does not receive the name of the reference that we
are currently iterating through. This isn't really a limiting factor, as
callers can simply pass the name via the callback data.

But this layout sometimes does make for a bit of an awkward calling
pattern. One example: when iterating through all reflogs, and for each
reflog we iterate through all refnames, we have to do some extra book
keeping to track which reference name we are currently yielding reflog
entries for.

Change the signature of the callback function so that the reference name
of the reflog gets passed through to it. Adapt callers accordingly and
start using the new parameter in trivial cases. The next commit will
refactor the reference migration logic to make use of this parameter so
that we can simplify its logic a bit.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>