Links to commits on Commits and Overview tabs of the cross-repository Pull Request may lead to 404 error in some cases.

Steps to reproduce

No write access to the target repo

1. Fork a repository you don't have write access to
2. Clone the fork on your local machine
3. Create a branch, create and push a commit(s)
4. Create a Pull Request from that branch to `master` of origin repo
5. Go to the Commits tab
6. All commit links will result in 404 when clicked
7. If you update the Pull Request with other commits, they will have broken links on both Commits and Overview tabs even if the entire page is reloaded unless you click on Diff tab

With write access to the target repo

1. Repeat aforementioned steps until creating the Pull Request
2. Create a PR to `master` of origin repo — you'll end up on Overview tab of Pull Request page, stay there
3. Create and push another commit(s) to the source branch
4. Click on Commits tab, but don't refresh entire page
5. Last added commit links will result in 404 when clicked
6. Same with the added commit links on Overview tab: they'll remain to be broken unless you click on Diff tab or reload entire page