### Please describe the problem.

TransferScanner crashes trying to add a file.

### What steps will reproduce the problem?

Start the web app.

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

4.20130709-g339d1e0 on Android.

### Please provide any additional information below.

There was a whole stack of nulls in some of those log lines as well. I've 

[[!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

TransferScanner crashed: unknown response from git cat-file ("refs/heads/git-annex:289/20f/SHA256E-s85883241--3bf01cfd6a422f9b661ed335e6142bbdaf899cd71587bb3cc812256064c7071e missing",refs/heads/git-annex:289/20f/SHA256E-s85883241--3bf01cfd6a422f9b661ed335e6142bbdaf899cd71587bb3cc812256064c7071e\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00(7981 more elided)\00.log)
[2013-07-16 15:19:26 NZST] TransferScanner: warning TransferScanner crashed: unknown response from git cat-file ("refs/heads/git-annex:289/20f/SHA256E-s85883241--3bf01cfd6a422f9b661ed335e6142bbdaf899cd71587bb3cc812256064c7071e missing",refs/heads/git-annex:289/20f/SHA256E-s85883241--3bf01cfd6a422f9b661ed335e6142bbdaf899cd71587bb3cc812256064c7071e\00\00\00\00\00(7991 more elided)\00.log)
# End of transcript or log.
"""]]

> Lacking other information about this problem, and with the assistant
> able to detect problems and run `git annex repair` to fix up corrupt git
> repos, I suppose this is [[done]] --[[Joey]]
