In the synchronizer: until the bug 270150 is solved, stop using protocols like...

In the synchronizer: until the bug 270150 is solved, stop using protocols like FISH, SFTP, SMB, etc. Check if the target or the source directory are empty

Before this change was applied, the bug caused problems if fish://, sftp://, or smb:// were directly used; however, using sshfs mounts (with e.g. sudo sshfs [...]), every synchronization seemed to work correctly, even using files that had one # (or several ones) in their names.

REVIEW: 124326
CCBUG: 270150