ObjectChecker: allow some objects to skip errors

ObjectChecker: allow some objects to skip errors

Some ancient objects may be broken, but in a relatively harmless way.
Allow the ObjectChecker caller to whitelist specific objects that are
going to fail checks, but that have been reviewed by a human and decided
the objects are OK enough to permit continued use of.

This avoids needing to rewrite history to scrub the broken objects out.

Honor the git-core fsck.skipList configuration setting when receiving a
push or fetching from a remote repository.

Change-Id: I62bd7c0b0848981f73dd7c752860fd02794233a6