[IMP] runbot_merge: logging when bumping timeout

I'd forgotten that in order to better handle cases where the CI is
highly backed up (and / or slow for some reason), we actually update
the CI timeout to really take the last "pending" status as the "true
start" of the CI. This might explain why lots of stagings needed extra
time: as of right now, out of 28835 stagings

- 20086 had their timeout bumped by more than 15mn
- 6967 had their timeout bumped by more than 30mn
- 264 had their timeout bumped by more than 1h
- 30 had their timeout bumped by more than 2h

Add some logging every time the CI is bumped this way, so we have
better visibility into that event.

Closes #429

runbot_merge/models/pull_requests.py

@@ -1670,6 +1670,7 @@ class Stagings(models.Model):
             vals = {'state': st}
             if update_timeout_limit:
                 vals['timeout_limit'] = fields.Datetime.to_string(datetime.datetime.now() + datetime.timedelta(minutes=s.target.project_id.ci_timeout))
+                _logger.debug("staging %s: got pending status, bumping timeout to %s (%s)", vals['timeout_limit'], cmap)
             s.write(vals)
 
     def action_cancel(self):