fsck: pull out function to check a set of blobs

fsck: pull out function to check a set of blobs

In `fsck_finish()` we check all blobs for consistency that we have found
during the tree walk, but that haven't yet been checked. This is only
required for gitmodules right now, but will also be required for a new
check for gitattributes.

Pull out a function `fsck_blobs()` that allows the caller to check a set
of blobs for consistency.

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