Additionally:
- replace some RunCommand(['git', ...]) calls with RunGit().
- replace Settings._GetConfig('rietveld.foo') _GetRietveldConfig('foo').
- merge and cache calls to git rev-parse --show-cdup.
- merge some calls to git merge-base.

Review URL: https://codereview.chromium.org/157913005

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@250248 0039d316-1c4b-4281-b951-d872f2087c98