Install new git hooks in this version.

This does beg the question of what to do if git later gets eg a
post-smudge hook, that could run git-annex smudge --update. I think the
thing to do in that case would be to make git-annex smudge --update
install the new hooks. That way, as the user uses git-annex, the hook
would be created pretty quickly and without needing any extra syscalls
except for when git-annex smudge --update is called.

I considered doing something like that for installation of the
post-checkout and post-merge hooks, which would have avoided the need
for v7. But the only place it was cheap to do it would be in git-annex smudge
which could cheaply notice that smudge.log didn't exist yet and so know
the hooks needed to be installed. But since smudge used to populate pointer
files, it would be quite surprising if a single git checkout/merge failed
to update the work tree, and so that idea didn't work out.

The other reason for v7 is psychological -- users don't need to worry
about whether they might be running an old version of git-annex that
doesn't support their v7 repository very well. And bug reports about
"v6" have gotten a bit of a bad association in my head since they often
hit one of the known limitations and didn't realize it was experimental.

newtyped RepoVersion Int to avoid needing 2 comparisons in
versionSupportsUnlockedPointers etc. Also it's just nicer.

This commit was sponsored by John Pellman on Patreon.