Allow users of ReceivePack access to the objects being sent

When implementing branch read access, we need to prove that the
newly created reference(s) point to objects that the user can see.

There are two ways that an object is reachable:
1)  It's reachable from a branch or change the user can see
2)  It was uploaded as part of the pack file the user sent us

This change adds additional methods in ReceivePack that will allow a
server to check the above conditions, in order to ensure that a user
is not trying to create a reference that they cannot see, or that a
malicious user isn't attempting to forge the SHA-1 of an object that
they cannot see in order to base a change off of it.

Change-Id: Ieba75b4f0331e06a03417c37f4ae1ebca4fbee5a