Previously, we would only update the default branch on a Geo secondary
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.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/9084