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

subcmds/list.py

@@ -35,6 +35,9 @@ This is similar to running: repo forall -c 'echo "$REPO_PATH : $REPO_PROJECT"'.
     p.add_option('-r', '--regex',
                  dest='regex', action='store_true',
                  help="Filter the project list based on regex or wildcard matching of strings")
+    p.add_option('-g', '--groups',
+                 dest='groups',
+                 help="Filter the project list based on the groups the project is in")
     p.add_option('-f', '--fullpath',
                  dest='fullpath', action='store_true',
                  help="Display the full work tree path instead of the relative path")
@@ -62,7 +65,7 @@ This is similar to running: repo forall -c 'echo "$REPO_PATH : $REPO_PROJECT"'.
       sys.exit(1)
 
     if not opt.regex:
-      projects = self.GetProjects(args)
+      projects = self.GetProjects(args, groups=opt.groups)
     else:
       projects = self.FindProjects(args)