Support filtering by group on forall and list subcmd

Enable operating against groups of repositories. As it stands, it isn't
compatible with `-r/--regex`.

`repo forall -g groupname -c pwd` will  run `pwd` for all projects in
groupname.

`repo forall -g thisgroup,-butnotthisone -c pwd` will  run `pwd` for all
projects in `thisgroup` but not `butnotthisone`.

`repo list -g groupname -n` will list all the names of repos in
`groupname`.

Change-Id: Ia75c50ce52541d1c8cea2874b20a4db2e0e54960