Raised when the revision under test is a merge commit.
A merge has no single "before" tree, so there is no well-defined set of
changes to assert on. On GitHub pull_request events the checked-out
HEAD is a synthetic merge of the PR into its base; assert on the PR's
individual commits instead (rev: with each SHA in
base.sha..head.sha), or on github.event.pull_request.head.sha.