Geo: Always update the default branch on the secondary
Previously, we would only update the default branch of Geo mirror if the default branch didn't match the primary default branch. However, the Gitaly FindDefaultBranch RPC may return an arbitrary branch if HEAD points to an invalid branch, so we can't trust that result to be correct.
To ensure that the default branch is always correct, we now write it out on the Geo secondary whenever we do a sync. This incurs an additional filesystem write, but that should be relatively minor.