Merged pull requests may show unrelated build statuses

Issue Summary

The pull request builds tab queries builds by branch name, not pull request ID. As a result, it may show builds that were performed on commits that were not actually part of the PR (built on the branch but not on commits related to the PR).

Steps to Reproduce

1. Create a PR and post some build statuses to the PR
2. Merge the PR but don't delete the source branch
3. Push more build statuses to the branch

Expected Results

• Old PR will only contain build statuses for that PR
• New PR from that branch will only show build statuses for that PR

Actual Results

• Old PR will show old and new build statuses
• New PR from that branch will show old and new build statuses

Workaround

• Don't re-use branch names across pull requests