Upgrade other repos than the current one by running git-annex upgrade
inside them, which avoids problems with upgrade code making assumptions
that the cwd will be inside the repo being upgraded.

In particular, this fixes a problem where upgrading a v7 repo to v8 caused
an ugly git error message.

I actually could not find a way to make Upgrade.V7 work properly
without changing directory to the remote. Once I got git ls-files to work,
the git cat-file failed because :path can only be used in the current git
repo.