For preprocess checks on pull requests, check against the base_ref of
the pull request (e.g. master) instead of against the seL4 revision in
the verification manifest.
If base_ref has advanced over the verification manifest revision, we do
not want to see that difference again on pull requests, but only the
difference the pull request causes itself.
Signed-off-by: Gerwin Klein <gerwin.klein@proofcraft.systems>
061017b242