### Please describe the problem.
After branching from master/HEAD^ and deleting the master branch, I was surprised that content introduced in master/HEAD is not reported as unused. Instead, one first has to delete the synced/master branch, and also the remote-tracking branches for master.

Would it make sense to discount synced/* and the remote tracking branches when gathering unused files? If the remote-tracking branch always reflects the state of the remote, tracking a branch should not cause annex to think that the local annex is somehow using all the files on the remote. Or would that be unsafe? As for the synced/* branches I'm not so sure.

### What steps will reproduce the problem?
Clone annex A to B.
Branch off of master^ in B.
Delete master.
Issue git annex unused. --> Content introduced in master is still not reported as unused.
Delete synced/master --> Cf. above.
Delete origin/master and origin/synced/master --> Content is now reported as unused.

### What version of git-annex are you using? On what operating system?

5.20131230