Double Travis builds

[SimonSapin]

r+ seems to make homu push to the auto branch twice, causing Travis-CI to schedule two builds (which take turns in the queue for available builders) while only the second one is useful.

[untitaker]

Can't reproduce. I just set up homu for a project of mine and Travis' build history only shows one entry for the auto branch. Perhaps you're looking at the PR build from Travis?

[SimonSapin]

CC @larsbergstrom

[larsbergstrom]

Travis ignores the first empty force-push to auto and only builds when the merge commit is pushed to that branch.

AppVeyor is the one where we're getting two builds right now (one for the empty push and then one for the merge commit).

[larsbergstrom]

cc @Manishearth

[jrmuizel]

It would be nice to fix this to reduce the load on the single macmini that we're now using for CI for WebRender.

[larsbergstrom]

cc @edunham for your queue :-)