borg: Start operation before mounting the repository location
The mount operation would run before registering the operation (which will show "Preparing backup" in the UI). This also means the backup button wasn't disabled during the entire mount, which was enabling users to start a backup multiple times and generally being quite confusing. Especially since some remote gvfs mounts can take quite a while.
Closes #392 (closed)
Edited by Fina Wilke