Add a `git annex hide $file` that behaves like drop, checking counter info
and updating location log to say the current repo no longer has a file --
but does not actually remove the content.

Then `git annex unused` can be used to clean it up later. And in the
meantime, it's still locally accessible. This can be useful if you're
planning to need to free up space later, but want to hold onto the content
for a while. Possibly you'll be disconnected later, so it's easier to push
out that intent now.

--

TODO:

* Make 100% sure this is safe. Drop, etc should never check content files
  are present on other repos if the location log doesn't say the repo
  has the content.

* What will `git annex get` do if it's asked to get a file that has been
  hidden?