Added: remove from data base repo that not exist

In fact if you have a removed repo on your disk it is still in the data base file (~/.gitdb).
When showing the full list of repo, it appears like a "not valid git repo".

I add a method clean_db to gitMeta which delete all not existing repo from the data base file and update the list of repo to be listed.

This method is used only when the there is no scan and the data base file exist.