BeginInteractiveRebaseForCommit is used for all the patch commands, and for
rewording. It works by setting the commit we want to stop at to 'edit'; this
doesn't work for merge commits. This wasn't a problem for the patch commands so
far, because you typically don't use custom patches with merge commits (although
we don't prevent this; maybe we should?).
However, it was a problem when you tried to reword a merge commit; this
previously failed with an error, as the test added in the previous commit
demonstrated.
Also, we want to add a new patch command that has to stop *before* the selected
commit (pull patch to new commit before the original one), and this wouldn't
work for the first commit in a feature branch, because it would have to set the
last commit before that to 'edit', which isn't possible if that's a merge (which
is likely).
To fix all this, use a 'break' before the selected commit if the commit is a
merge. It is important that we only do it in that case and not always, otherwise
we would break the new regression tests that were added a few commits ago.
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