### Please describe the problem.

I'm using git-annex with the [calibre e-book library](https://calibre-ebook.com/) software. Sometimes calibre will rename a directory. After a directory rename git annex unlock no longer works. It works once I've committed the changes to git.

It would be nice if git-annex could give an error to explain why the unlock fails. I'd even be happy with an extra flag to unlock, like --debug or --verbose to show the message.

### What steps will reproduce the problem?

1. create a directory inside a repository
2. save a file in the directory
3. add the file to git annex and commit
4. rename the directory
5. try unlocking the file

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

6.20161012 on Debian

### Please provide any additional information below.

[[!format sh """
# If you can, paste a complete transcript of the problem occurring here.
# If the problem is with the git-annex assistant, paste in .git/annex/daemon.log

    ~/scratch$ mkdir annex
    ~/scratch$ cd annex/
    ~/scratch/annex$ git init
    Initialized empty Git repository in /home/edward/scratch/annex/.git/
    ~/scratch/annex (master)$ git annex init --version=6
    init  ok
    (recording state in git...)
    ~/scratch/annex (master)$ mkdir foo
    ~/scratch/annex (master)$ cd foo
    ~/scratch/annex/foo (master)$ echo test > test
    ~/scratch/annex/foo (master)$ git annex add test
    add test ok
    (recording state in git...)
    ~/scratch/annex/foo (master)$ git commit -m 'test'
    [master (root-commit) 6368036] test
     1 file changed, 1 insertion(+)
     create mode 120000 foo/test
    ~/scratch/annex/foo (master)$ cd ..
    ~/scratch/annex (master)$ mv foo bar
    ~/scratch/annex (master)$ cd bar
    ~/scratch/annex/bar (master)$ git annex unlock test
    ~/scratch/annex/bar (master)$ 

# End of transcript or log.
"""]]

### Have you had any luck using git-annex before? (Sometimes we get tired of reading bug reports all day and a lil' positive end note does wonders)

git-annex is amazing, I use it all the time. Thanks!