Fixed an issue that was caused by the merge of ‘improve/default_branch’.

Not sure how to add a test for this, but update_attributes can’t be used anymore,
since the default_branch is no longer in the db. This issue is manifest be a 500
error when visiting a newly created project page (or a project with an empty repo).

The commit in question is 253680bbffe2423a11d4fa8626c2b35c289ea8cc.