Accept git repository URLs for the control repository

[smashwilson]

I shouldn't expect casual contributors to have the control repository cloned and on their filesystem. Allow it to be specified as a cloneable git URL and a branch name, do the clone myself to a directory under ~/.deconst, and link that.

[kenperkins]

Perhaps allow a per-deployment packaging that pre-populates so that end users don't have to worry about either control or content repositories.