init: missing parameter to avoid error if repository already initialized

[paolobenve]

There isn't any way to avoid the error when trying to initialized an already initialized repository

Use case: I use a script with attic to backup all of my mysql db's, and when a new db is created I must initialize the repository by the backup script. So by now the only way is to init the repository every time I'm going to create an archive in it. It works, because the init command doesn't make anything on an existing repository, but it's far from elegant