files_transaction_commit(): clean up empty directories

files_transaction_commit(): clean up empty directories

When deleting/pruning references, remove any directories that are made
empty by the deletion of loose references or of reflogs. Otherwise such
empty directories can survive forever and accumulate over time. (Even
'pack-refs', which is smart enough to remove the parent directories of
loose references that it prunes, leaves directories that were already
empty.)

And now that files_transaction_commit() takes care of deleting the
parent directories of loose references that it prunes, we don't have to
do that in prune_ref() anymore.

This change would be unwise if the *creation* of these directories could
race with our deletion of them. But the earlier changes in this patch
series made the creation paths robust against races, so now it is safe
to tidy them up more aggressively.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>