Pulls the logic for computing an issue's "status" - has LGTM, is in
CQ, etc. - out of the "git cl status" command itself, so that other
tools can get the status of a Changelist.

BUG=379849
R=iannucci@chromium.org

Review URL: https://codereview.chromium.org/555973005

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@291928 0039d316-1c4b-4281-b951-d872f2087c98