git fetch origin $SHA1 doesn't work as often as it could

So I'm trying to make cloning the rust repo a little easier, and I noticed there's a new feature to `git submodule update --depth=1` -- but it doesn't work for this, even though it could.

Reason is that fetch-pack will only fetch by sha1 if the remote specifies allow_tip/unreachable_sha1, even if the sha1 specified matches an advertised ref.

My thought is to rework filter_refs() to match on oid if matching on name fails. Thoughts?

- JP