Jenkins build status not visible in big PRs

[phillipj]

With nodejs/node#9745 which has 100+ commits, we just found that the bot pushes Jenkins build status to the wrong commit SHA.

Probably because the list of commits fetched in findLatestCommitInPr() is paginated by GitHub, and therefore we'll have to do some pagination tricks to ensure we actually fetch the latest commit of that specific PR, even if it consists of hundreds of commits.

[jbergstroem]

ugh :/