See PHI1860.
See <https://discourse.phabricator-community.org/t/arcanist-says-the-revision-does-not-merge-cleanly-into-master-even-after-cleanly-merging-master/4184/>.
These reports describe issues where `arc land` fails to merge changes which should merge. (PHI1860 may or may not describe the same issue as the Discourse report.)