Expand presubmit_support and gclient_scm to support
getting basic status and changed files from a Git
checkout.

BUG=5339
TEST=presubmit_support.py works from a git checkout,
gcl presubmit works from a svn checkout
Review URL: http://codereview.chromium.org/242140

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@28092 0039d316-1c4b-4281-b951-d872f2087c98