This addresses the
which-func-ff-hook error: (void-function magit-section--backward-find)
seen when `magit-status' is run in a buffer with imenu support on a
fresh Prelude checkout using the sample `prelude-pinned-packages.el'.
In magit 3.3.0, magit-section was spun off into its own package.
Later, magit commit 4727dcb removed the `magit-section--backward-find'
from magit-section.el. Therefore, unless magit and magit-section are
in the same pinned or unpinned state the above error will occur.
