Ask the wiki repo, not Gollum, if it's empty

We need to skip empty repositories when creating a backup. Before this
change, we were asking gollum-lib if the wiki contains any _pages_. Now
we ask gitlab_git if the repository contains _files_. This should
resolve gollum_lib Grit timeouts in the backup script.
Merge branch 'empty_wiki_backup_check' into 'master'

Empty wiki backup check

We need to skip empty repositories when creating a backup. Before this
change, we were asking gollum-lib if the wiki contains any _pages_. Now
we ask gitlab_git if the repository contains _files_. This should
resolve gollum_lib Grit timeouts in the backup script.

See merge request !1195