Fork PRs that have had PRs merged into them cause the release to fail

[zephraph]

Describe the bug

artsy/reaction#2730 is a PR that was created from a fork. That fork had a PR (pepopowitz/reaction#3) merged into it (from another fork).

Now releases are broken because it's trying to look up a PR at artsy/reaction/pull/3 when it should be looking up the PR from pepopowitz/reaction/pull/3

Additional context

Here's the CI build: https://circleci.com/gh/artsy/reaction/42174

[hipstersmoothie]

related: #465

[hipstersmoothie]

Other related bugs:

1. commit with add auto release to CI #1 has been merged, a second message with add auto release to CI #1 is merged and it thinks add auto release to CI #1 has been released again
2. commit with Deprecate --major --minor --patch flags #100 merged, repo has < 100 issues, results in 404
3. merging back and forth from 2 prs, potentially many repeated pr numbers

[adierkens]

🚀 Issue was released in v7.3.5 🚀