Pull request commit on forks has the wrong link when merged into master

If you have a repo A, then you fork A (call the forked repo B), then merge B back into A, the links for the pull requests on commits to B will be incorrect.

More specifically, if you merge PR#3 into B, then it has a commit that says (PR#3) and has a link for the third pull request of B. However, if you then merge B into A, the commit still says (PR#3), but it has no memory of the earlier repo and links to the third pull request of A.