Ignore changes to un-related files in regex matches. (#81)

## Summary:
If there were changes to unrelated files, such as in a big diff against a parent branch, then folks could get pulled in to a bunch of unrelated PRs, which is a big hassle.

Issue: FEI-5521

## Test plan:
Tests are passing, hopefully that's enough to confirm that this is working!

Author: jeresig

Reviewers: somewhatabstract

Required Reviewers:

Approved By: somewhatabstract

Checks: ✅ Jest Coverage, ✅ Jest, ✅ Eslint, ⏭  autofix, ✅ build_index, ✅ gerald, ✅ lint_and_unit

Pull Request URL: #81