gitlab handler fails to send status immediately, causing confusion in gitlab gui & premature merges

When buildbot's gitlab http endpoint receives a webhook that will trigger builds,
it currently does not send any status for those builds until they have actually started.

This causes two problems:

- the gitlab gui shows an error message until at least one status has been recieved
- the gitlab gui, if you have checked 'merge when build succeeds', will merge prematurely if a build is still pending but all started builds have finished