This introduces python_git_runner.sh, and makes git-cl, git-cache, git-number,
and git-try use it.

R=agable@chromium.org, szager@chromium.org
BUG=261738

Review URL: https://codereview.chromium.org/188383002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@256131 0039d316-1c4b-4281-b951-d872f2087c98