ignore git fsck error

example: [behat/behat](https://github.com/Behat/Behat/issues/341)
```
$ git fsck
Checking object directories: 100% (256/256), done.
error in tree 7708dad681f44c1d69c369c9b1bd55832ee4eb52: duplicateEntries: contains duplicate file entries
Checking objects: 100% (26442/26442), done.
```
