teach core.logallrefupdates an "always" mode

When core.logallrefupdates is true, we only create a new
reflog for refs that are under certain well-known
hierarchies. The reason is that we know that some
hierarchies (like refs/tags) do not typically change, and
that unknown hierarchies might not want reflogs at all
(e.g., a hypothetical refs/foo might be meant to change
often and drop old history immediately).

However, sometimes it is useful to override this decision
and simply log for all refs, because the safety and audit
trail is more important than the performance implications of
keeping the log around.

This patch introduces a new "always" mode for the
core.logallrefupdates option which will log updates to
everything under refs/, regardless where in the hierarchy it
is (we still will not log things like ORIG_HEAD and
FETCH_HEAD, which are known to be transient).