refs/files: introduce function to perform normal ref checks

refs/files: introduce function to perform normal ref checks

In a subsequent commit we'll introduce new generic checks for direct
refs. These checks will be independent of the actual backend.

Introduce a new function `refs_fsck_ref()` that will be used for this
purpose. At the current point in time it's still empty, but it will get
populated in a subsequent commit.

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