post a github status when build is submitted, not just started

[mikesherov]

In environments where the duration between submission and start is large, it looks like Mergeatron hasn't done anything for a long time. Mergeatron should update the github status as soon as the build is submitted.

[steves]

The only tricky part with this is that we don't get the ID of the Jenkins job until after we submit it. In that case the merge status could link to Jenkins but not to the actual build that's running.