Implement github pipeline req of current-patchset

Require that the commit from the event is the latest commit in the pull
request.

Also fix a problem with faked github status grabs. Now we're sending an
event where the sha of the event isn't the head sha, and that was
tripping up our fakes.

Change-Id: I269c97d096e42f0a2d4a0f1b0e57eb238e0b7baf
6 files changed