### Please describe the problem.

Both my Android devices where not processing git-annex updates due to stale lock files. While the lock files are different, I've reported them both together as they are related. 

### What steps will reproduce the problem?

Unknown, perhaps the assistant crashed, or the battery ran flat on them.

To resolve the issue I had to manually remove the lock files.

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

On my Android phone, daily build 4.20130614-g221aea4
On my Android tablet, daily build 4.20130621-g36258de

### Please provide any additional information below.

It seems to me that it'd be useful to have the assistant check to see if the lock files are still valid and remove them if they're stale.

[[!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

My phone:

From ssh://git-annex-flick-andrewannex_phonecamera/~/phone-camera
   987dc25..682cdd1  git-annex  -> flick_phonecamera/git-annex
fatal: Unable to create '/storage/emulated/legacy/DCIM/.git/refs/remotes/flick_phonecamera/synced/git-annex.lock': File exists.

My tablet:

Committer: Adding Coleman-C..eedom.pdf
Committer: Committing changes to git
fatal: Unable to create '/mnt/sdcard/reference/.git/index.lock': File exists.

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

> The '/mnt/sdcard/reference/.git/index.lock' lock file will now be
> automatically dealt with. Have not done anything about the refs/remotes
> lock files yet. --[[Joey]]
> 
> Now the assistant deals with all stale git lock files on startup. 
> [[done]] --[[Joey]]