cleanup: Check parents of internal refs
Currently we check if any of our internal references are present on the
user-provided commit-map file. However,
refs/merge-requests/<MR_IID/merge refs are merge commits created
internally by GitLab and are not present on client copies of the
repository. Consequently, they will not be present on the commit-map,
but they may have parent commits that are children of a rewritten commit
and we will fail to remove all of the internal references that are
keeping a large blob alive.
To resolve this, update our git-for-each-ref(1) format string to include both the object referenced by the reference and its parents. If any of these are present on the commit-map then we delete the reference.