Don't call apt-get update when there were no changes in repo structure

This supports PPA repo definitions on Ubuntu only, but I suspect is the majority of use cases.  It's not obvious without quite a bit of work how to get a more general use case for mod_repo in apt.py to work (Debian or secure ppa_auth definitions).

Also, a small fix to make get_repo a little less finicky.  

This should fix issue 6866.
https://github.com/saltstack/salt/issues/6866