Refactoring: Extract helper functions from CMDtry in git_cl.py.

The purpose of this change is to prepare for modifying git cl try
so that builders on multiple masters can be triggered in one invocation
(http://crbug.com/640740).

This should not affect behavior; this CL makes several non-essential
changes to formatting and comments.

Review-Url: https://codereview.chromium.org/2442153002