Default ref-update batch to 5000; make it env-tunable

GitHub returns 500 Internal Server Error when a single push updates ~10k refs at once but accepts 5k — well below entire-server's 25k cap. Lower the default batch to 5_000 so mirroring a many-ref repo works against GitHub out of the box, and add GITSYNC_MAX_REF_UPDATES_PER_PUSH to raise it for targets known to tolerate larger pushes (entire-server, up to 25k) and cut round trips.