Add fetch for vc, update show-branches

Add fetch for vc, update show-branches

Fetch bound on `f' in vc-map and vc-dir-map.  Does not check if `backend' is
anything other than `Git'. Calls `fetch --all --verbose', or with arg, prompts
for location from which to fetch.

re/vc-show-branches now takes an optional arg, which will enable showing of
remote branches.