per file merge hooks would like a way to maintain state

Now that bug #491711 has landed, using it has shown a small deficiency.

Specifically, per-file hooks often want to maintain some state across the merge
(such as whether they are enabled, what files to work on, etc.)

One option would be to change the hook so that at the beginning of the merge
we call a hook which returns an object (potentially None if the hook is
disabled). Afterwards, in the inner loop, those objects are used (methods
called) that perform the same actions that we are doing today.