Large PR with merge conflicts doesn't automatically fail

[aheuermann]

We had an odd case today where a rather large PR had merge conflicts but the dotci job still ran. Typically the job fails on checkout at git fetch origin '+refs/pull/<PR #>/merge:', but in this case the job ran, with what I assume was old code.

It looks like the github api determines merge-ability asynchronously. Which I'm wondering if this is also what updates refs/pull/<PR #>/merge, but not sure...

Doing some digging, other jenkins github plugins don't look at mergeable or mergeable_state, but I did notice that travis-ci loops and checks before triggering the build.

Anyway, I think the build should fail if there are merge conflicts. Do you think it is worth adding a similar check to dotci?