enable filter.annex.process in v9

This has tradeoffs, but is generally a win, and users who it causes git add to
slow down unacceptably for can just disable it again.

It needed to happen in an upgrade, since there are git-annex versions
that do not support it, and using such an old version with a v8
repository with filter.annex.process set will cause bad behavior.
By enabling it in v9, it's guaranteed that any git-annex version that
can use the repository does support it. Although, this is not a perfect
protection against problems, since an old git-annex version, if it's
used with a v9 repository, will cause git add to try to run
git-annex filter-process, which will fail. But at least, the user is
unlikely to have an old git-annex in path if they are using a v9
repository, since it won't work in that repository.

Sponsored-by: Dartmouth College's Datalad project