Migrate to SHA-256 and implicit repo ID (PARTIAL REPO FORMAT CHANGE)

Migrate to SHA-256 and implicit repo ID (PARTIAL REPO FORMAT CHANGE)

* local, rsync, ssh, sftp repositories are still compatible
* gitception/git backend repositories are not compatible and need to be
  deleted and recreated
* Put manifest in a static location, so we don't need #fragment in the URL
* Record repository ID for each remote, and warn if it changes.
* Use SHA-256 by default but allow reading SHA-224-identified packfiles
* The URL #fragment identifies branch to use when using the git backend