pkg_in_pipe: keep PRs merged by rebase or squash

[glehmann]

like xcp-ng-rpms/blktap#28

At the moment, this is not the case which causes the corresponding PR to be missing from the generated report.

This PR is part of a tree containing 4 PRs:

1. master
2. "pkg_in_pipe: keep PRs merged by rebase or squash" (this PR) → master
3. pkg_in_pipe: write a machine readable version of the report #839 → pkg_in_pipe: keep PRs merged by rebase or squash #838
4. pkg_in_pipe: move the plane cards of the builds of a tag to a given state #840 → pkg_in_pipe: write a machine readable version of the report #839
5. pkg_in_pipe: generate a draft of the release post from the report #841 → pkg_in_pipe: write a machine readable version of the report #839