gclient_tests: add GitWrapper test cases

gclient_scm: make RunGit private and add url check

gclient_scm: add revert, run_hooks, and status

gclient_scm: add git implementations of cleanup, diff, and export

gclient_scm: implment GitSCM::update

gclient_scm: add a checks for unsupported scm and unimplemented method

Review URL: http://codereview.chromium.org/235005

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@27241 0039d316-1c4b-4281-b951-d872f2087c98