Fix busy loop when "Cancel build on update" is enabled

[eagletmt]

ghprb enters busy loop with the following steps.

1. Update pull-request X and start a build X
2. Update pull-request Y in the same repository and start a build Y
   • Build Y goes to the build queue since build X is currently running
3. Update pull-request X and start a build X'
   • GhprbCancelBuildsOnUpdate#cancelCurrentBuilds() is called
   • Check if queuedItem is cancellable, but it turns out impossible
     because prId is different
   • project.getQueueItem() is called repeatedly but it doesn't return
     null until build X finishes and build Y starts.
   • At the same time, build X' doesn't go to the build queue because
     it's blocked by GhprbCancelBuildsOnUpdate#onScheduleBuild()

ghprb should use Queue#getItems API to find cancellable builds in the
build queue.

[eagletmt]

@samrocketman Could you have a look?

[samrocketman]

The code looks good. Have you had a chance to test this @eagletmt ?

[bjoernhaeuser]

@eagletmt this looks like an important fix, though we need your contribution to actually test this. How can this be tested? Were you able to confirm that your patch works?

[bjoernhaeuser]

@eagletmt @samrocketman How can we proceed here? :)

[bjoernhaeuser]

I have tested this manually locally and the new code works. Though I was not able to reproduce a real bug, this looks to me like an optimization which is just improving the current code.

@eagletmt is this observation correct?