### Please describe the problem.

git-annex webapp won't run 

### What steps will reproduce the problem?

[[!format sh """
arthur@machine:~/annex$ git-annex --debug webapp
[2013-07-29 15:02:15 CEST] read: git ["--git-dir=/home/arthur/annex/.git","--work-tree=/home/arthur/annex","show-ref","git-annex"]
[2013-07-29 15:02:15 CEST] read: git ["--git-dir=/home/arthur/annex/.git","--work-tree=/home/arthur/annex","show-ref","--hash","refs/heads/git-annex"]
[2013-07-29 15:02:15 CEST] read: git ["--git-dir=/home/arthur/annex/.git","--work-tree=/home/arthur/annex","log","refs/heads/git-annex..a2b8f10ef258dff1a91e0354b2e2a58241631c9a","--oneline","-n1"]
error: object file /home/arthur/annex/.git/objects/a2/b8f10ef258dff1a91e0354b2e2a58241631c9a is empty
fatal: loose object a2b8f10ef258dff1a91e0354b2e2a58241631c9a (stored in /home/arthur/annex/.git/objects/a2/b8f10ef258dff1a91e0354b2e2a58241631c9a) is corrupt
[2013-07-29 15:02:15 CEST] read: git ["--git-dir=/home/arthur/annex/.git","--work-tree=/home/arthur/annex","log","refs/heads/git-annex..8d4b8e04ccf0092d625f680b42e73d7bf15c6517","--oneline","-n1"]
error: object file /home/arthur/annex/.git/objects/a2/b8f10ef258dff1a91e0354b2e2a58241631c9a is empty
fatal: loose object a2b8f10ef258dff1a91e0354b2e2a58241631c9a (stored in /home/arthur/annex/.git/objects/a2/b8f10ef258dff1a91e0354b2e2a58241631c9a) is corrupt
[2013-07-29 15:02:15 CEST] read: git ["--git-dir=/home/arthur/annex/.git","--work-tree=/home/arthur/annex","log","refs/heads/git-annex..6b2665208c11c9ecf969294bf45baac31894d8a7","--oneline","-n1"]
error: object file /home/arthur/annex/.git/objects/a2/b8f10ef258dff1a91e0354b2e2a58241631c9a is empty
fatal: loose object a2b8f10ef258dff1a91e0354b2e2a58241631c9a (stored in /home/arthur/annex/.git/objects/a2/b8f10ef258dff1a91e0354b2e2a58241631c9a) is corrupt
[2013-07-29 15:02:15 CEST] read: git ["--git-dir=/home/arthur/annex/.git","--work-tree=/home/arthur/annex","log","refs/heads/git-annex..9d8429668f2148ea43760fb430e5950fbf42751e","--oneline","-n1"]
error: object file /home/arthur/annex/.git/objects/a2/b8f10ef258dff1a91e0354b2e2a58241631c9a is empty
fatal: loose object a2b8f10ef258dff1a91e0354b2e2a58241631c9a (stored in /home/arthur/annex/.git/objects/a2/b8f10ef258dff1a91e0354b2e2a58241631c9a) is corrupt
[2013-07-29 15:02:15 CEST] chat: git ["--git-dir=/home/arthur/annex/.git","--work-tree=/home/arthur/annex","cat-file","--batch"]

git-annex: fd:14: hGetLine: end of file
failed
[2013-07-29 15:02:15 CEST] read: ssh ["-O","stop","-S","/home/arthur/annex/.git/annex/ssh/arthur@git-annex-hostname-arthur_annex","-o","ControlMaster=auto","-o","ControlPersist=yes","arthur@git-annex-hostname-arthur_annex"]
git-annex: webapp: 1 failed
"""]]


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


$ git-annex version
git-annex version: 4.20130516.1
build flags: Assistant Webapp Pairing Testsuite S3 WebDAV Inotify DBus XMPP
local repository version: 3
default repository version: 3
supported repository versions: 3 4
upgrade supported from repository versions: 0 1 2

$ git --version
git version 1.7.9.5


### Please provide any additional information below.