Properly fix wiki restore
My previous patch didn't fix the restore issue (!247 (merged)).
ProjectWiki.new() creates a new wiki git repository, so any tries to bare clone a bundle fail. With this patch we remove the newly created wiki.git before restoring from the backup bundle.
\cc @jacobvosmaer @dzaporozhets are you ok with this solution/hack?
Relevant issues: #845 (closed)