refs/files: extract function to check single ref

refs/files: extract function to check single ref

When checking the consistency of references we create a directory
iterator and then verify each single reference in a loop. The logic to
perform the actual checks is embedded into that loop, which makes it
hard to reuse. But In a subsequent commit we're about to introduce a
second path that wants to verify references.

Prepare for this by extracting the logic to check a single reference
into a standalone function.

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