graft: fix collision detection with origin revisions that are missing When grafting something with a matching origin, it would normally be skipped: skipping already grafted revision 123 (23 also has origin 12) But after stripping a graft origin, graft could fail with a reference to the origin that no longer exists: abort: unknown revision '5c095ad7e90f871700f02dd1fa5012cb4498a2d4'! Instead, detect that the origin is unknown and skip it anyway, like: skipping already grafted revision 8 (2 also has unknown origin 5c095ad7e90f871700f02dd1fa5012cb4498a2d4)