Improve function to find PR from commit status

This function used to use a costly search to find all the PRs that have
the mentioned commit in them. This can chew through the relatively small
search API limit quickly. Instead, we can use repo information that
comes in the status to narrow down which repository to care about, thus
we can just get a listing of PRs in that repository and find the PR
where the mentioned sha is the HEAD of the PR.

Change-Id: I42941366bb450d53ebf23d9bead603210f51ed57
Signed-off-by: Jesse Keating <omgjlk@us.ibm.com>
2 files changed