Fetch values ftom the underlying Config::Versioned repository.
the commit id of the head is written into the local version property and all further queries are done against this commit id.
You can set the version to be used at any time by passing the commit id (sha1 hash) to
To advance to the head commit of the underlying repository,
fetch_head_commit to get the id of the head and set it using
Receive the sha1 commit id of the topmost commit of the underlying repository.