Update cros_workon to use repo list instead of repo forall.
Update cros_workon to use repo list instead of repo forall.

repo list is a little faster and is more idiomatic.

BUG=chromium-os:5932, chromium-os:6126
TEST=Make sure cros_workon info --all and cros_workon info kernel still work

Change-Id: I61dfdc8a00669e88b90298f7c990a497b61c174a
Reviewed-on: http://gerrit.chromium.org/gerrit/877
Reviewed-by: Zdenek Behan <483cf2a9a4e071d4fd973db6a5a78c40f490667c@chromium.org>
Tested-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>

marineam/coreos-dev-util,coreos/dev-util,marineam/coreos-dev-util,coreos/dev-util,mgireesh05/dev-util,mgireesh05/dev-util,coreos/dev-util,mgireesh05/dev-util