In the `delaysmudge` branch, I've implemented the delayed worktree update in
the post-merge/post-checkout hooks for v6. It works very well!

In particular, with annex.thin set, checking out a branch
containing a huge unlocked file does a fast hard link to the file.

Remaining problem before merging that is, how to get the new hooks installed? Of
course `git annex init` and `git annex upgrade` install them, but I know plenty
of people have v6 repositories already, without those hooks.

So, would it be better to bump up to v7 and install the hooks on that upgrade,
or stay on v6 and say that it was, after all, experimental up until now, and
so the minor bother of needing to run `git annex init` in existing v6 repositories
is acceptable? If the version is bumped to v7, that will cause some pain
for users of older versions of git-annex that won't support it, but those old
versions also have pretty big gaps in their support for v6. I'm undecided,
but leaning toward v7, even though it will also mean a lot of work to update
all the documentation, as well as needing changes to projects like datalad that use
git-annex. Feedback on this decision is welcomed below...
