v6 git-annex unlock

v6 git-annex unlock

Note that the implementation uses replaceFile, so that the actual
replacement of the work tree file is atomic. This seems a good property to
have!

It would be possible for unlock in v6 mode to be run on files that do not
have their content present. However, that would be a behavior change from
before, and I don't see any immediate need to support it, so I didn't
implement it.