Since 94e03f5ac8, we lazily filter out non-heads from `View`'s set
of head. Accessing the `MutRepo::view()` triggers that filtering. This
patch makes `DescendantRebaser` not unnecessarily do that. That speeds
up the rebasing of 162 descendants in the git.git repo from ~3.6 s to
~330 ms. Rebasing 1272 descendants takes ~885 ms.