Repack git repositories daily

The volume of loose refs which get replicated to our git backends
pile up quickly during periods of heavy use, which increases load
and network traffic on the servers, slowing down updates and raising
the risk from network errors. Reduce the scheduled repack frequency
from weekly to daily, which should also result in quicker repacks.

Change-Id: I6380e67444c54f5b1a1c1b5b631e95c5a20f119a
Merge "Repack git repositories daily"