Currently, after a push we trigger the 'push' system hook on the
secondary for every ref. This hook performs `git fetch primary
--force`. When a repository with a large number of local refs is pushed
for the first time, git will then try to fetch the entire repository
once for every ref, generating a large amount of network and disk usage
for the temporary packfiles used during the fetch process.

In addition to this, if a fetch fails in the
`GeoRepositoryUpdateWorker`, its temporary packfile will remain present
until the next GC run.

To work around this, define a new 'fetch' hook type. Fake the system
hooks so that this is only called for secondary Geo nodes. Then remove
the fetching from the per-ref update step, and explicitly call the hook
once per push instead.

This means that there is a risk the update step will happen before the
fetch is finished, but it does stop the disk usage problem.