### Please describe the problem.

On my bare git-annex repo, `git annex fsck -q` reports:

    ** No known copies exist of SHA256E-s1212237--d2edd369f6a9005e23f022c7d797b166c66b90defc561329dbafab9a0fc0c7fc.jpg

`git log -SSA256E...` returns nothing. `git annex repair` and `git annex forget` do not fix the problem.

This means that running `fsck` from cron or a script will now always fail. There should be a way to recover from this situation.

### What steps will reproduce the problem?

According to IRC this "can happen if you annexed a file and then deleted it without ever committing to git".


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

5.20140717 from Ubuntu utopic

[[!tag confirmed]]

> [[done]] -- fsck --all or in a bare repo will skip over keys that have
> been marked dead, which can be done with `git annex dead --key`.
> 
> Might go further later, and make `git annex drop --force`, when removing
> the last copy of a key, mark it as dead. But maybe that is too DWIM.
> --[[Joey]]
