supermirror branch scanner should record last scanned timestamp

[David Allouche]

The branch scanner should record the time it last processed a branch as Branch.last_scanned.

This would enable two things:

* fix the silly "this branch was not scanned yet, or no revision were found on the last scan" message on the branch page.
* optimise the branch scanner to only process branches whose last_scanned < last_mirrored.

[David Allouche (ddaa)]

The Branch.status_updated timestamp should probably be renamed to last_scanned.

[James Henstridge (jamesh)]

On IRC, Robert suggested storing the last scanned revision ID. This may be more useful in debugging problems with the scanning.

Recording the last scan timestamp is probably a good idea still, since it might be more meaningful to users.

As far as the "last_scanned < last_mirrored" optimisation, it might be worth using revision IDs here and only scan if last_scanned_id != last_mirrored_id. This would require branch puller changes though.

[James Henstridge (jamesh)]

There was a problem with the fix for this bug that was not caught by the test suite. The bug and the problem with the test suite have been fixed in rocketfuel. I'll close the bug when the patch gets cherrypicked or a new rollout happens.