fix: backfill files by commit for rebased PRs

[lukekarrys]

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Fixes #2140 🦕

This has the effect of not associating a PR with any of the commits from a PR that is rebased and merged. Previously it would associate the PR only with the first commit, but this would also add all the files from the PR to that first commit.

A different (or future) solution would be to decouple connecting PRs and files with commits. I considered that but thought that solution would increase the scope too much. I'm open to reworking the PR in that direction though.

[lukekarrys]

This has the effect of not associating a PR with any of the commits from a PR that is rebased and merged.

After writing that comment, I think I found a simple solution to this. I added another commit to this PR that always sets commit.pullRequest if there are any associatedPullRequests.

I can revert this commit if this is not a desired change, or make it a separate PR.

I would love for this additional change to land since in the npm CLI we always run an additional API request for each commit to get its associated PRs for this reason.