Merge branch 'mh/expire-updateref-fixes'

Various issues around "reflog expire", e.g. using --updateref when expiring a reflog for a symbolic reference, have been corrected and/or made saner. * mh/expire-updateref-fixes: reflog_expire(): never update a reference to null_sha1 reflog_expire(): ignore --updateref for symbolic references reflog: improve and update documentation struct ref_lock: delete the force_write member lock_ref_sha1_basic(): do not set force_write for missing references write_ref_sha1(): move write elision test to callers write_ref_sha1(): remove check for lock == NULL