Make sure that layman has an up-to-date cache.

Layman always needs it's cache files, otherwise it can't find the repos:

 * Adding overlay,...
 * Warning: an installed db file was not found at: ['/var/lib/layman/cache_ac494f50f5736be7871962c0dec7b3bb.xml', '/var/lib/layman/cache_da9e0ccd5c4b933cbaf910ea709c3767.xml']
 * Exception: Overlay "foss-cloud" does not exist.

 * CLI: Errors occurred processing action add
 * Exception: Overlay "foss-cloud" does not exist.
Do not add empty commit for portage repos (bug #201)

We currently setup /usr/portage and /../portage-overlay as empty repos
with a single commit which is required to branch. This leads to a merge
on the first pull which is unnecessary and requires manual intervention.

The solution is to not commit anything and use --orphan to create a
truly empty branch (when not using the master branch).