Celery branch scan jobs intermittantly fail to show revisions for new branches

Repeated pushes of the same local branch to new names on staging show that sometimes new revisions are shown, and sometimes not. ~abentley/+junk/foo49-foo52 all have the same contents, but 49-51 show new revisions, while 52 does not.

This appears to be due to races. The last scan of branch 52 reads:
[2012-04-16 19:23:55,669: INFO/PoolWorker-4] Adding 0 new revisions.
[2012-04-16 19:23:55,669: INFO/PoolWorker-4] Deleting 12 branchrevision records.
[2012-04-16 19:23:55,674: INFO/PoolWorker-4] Inserting 12 branchrevision records.
[2012-04-16 19:23:55,937: INFO/PoolWorker-4] Updating branch scanner status: 12 revs
[2012-04-16 19:23:55,982: INFO/MainProcess] Task lp.services.job.celeryjob.CeleryRunJob[26ab04ab-a295-46bb-9245-14af1af88ed4] succeeded in 0.483397006989s: None

The last scan of foo51 reads:
[2012-04-16 19:23:38,794: INFO/PoolWorker-2] Adding 0 new revisions.
[2012-04-16 19:23:38,794: INFO/PoolWorker-2] Deleting 0 branchrevision records.
[2012-04-16 19:23:38,794: INFO/PoolWorker-2] Inserting 12 branchrevision records
.
[2012-04-16 19:23:39,088: INFO/PoolWorker-2] Updating branch scanner status: 12
revs

This issue is not critical because it affects only staging, not production.

This issue is likely visible because an initial push will create two BranchScanJobs, increasing the likelihood of concurrent jobs. The current scan_branches script does not run jobs in parallel.