Accessing the repository's config and immediately taking a snapshot of
it is a common operation, so let's provide a convenience function for
it.

This way we can assume we have a consistent view of the config situation
when we're looking up remote, branch, pack-objects, etc.

We look up a reference in order to figure out if it's the current
branch, which we need to free once we're done with the check.

As a bonus, only perform the check when we're passed the force flag, as
it's a useless check otherwise.