`git annex unused` only looks at tags and branches. Some users would like
to drop any objects that are not pointed to by any commit, but keep any
object that any commit ever referenced.

This could be a switch, like --ever.

The implementation would need to walk the history of all branches and check
all commits. This would tend to be slow. It could look at tags+branches as
it does now as a first pass, and only do the slow part if there are objects
not referred to by the tags+branches. And, it could stop looking through
the whole commit history if there were no more objects to check. Still,
gonna be slooow. Another optimisation would be to get only the objects
changed by the commit, and not look at the whole tree as it appeared on
each commit. --[[Joey]]
