Properly fix wiki restore.

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.
Merge branch 'backup_restore' into 'master'

Properly fix wiki restore

My previous patch didn't fix the restore issue (!247).

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

See merge request !248