Rename git-utils to git_utils so it can be imported with python.

While this is not yet necessary, it could be soon and I prefer to do this before
too many people uses it since it breaks everyone.

R=cmp@chromium.org,groby@chromium.org
BUG=
TEST=


Review URL: https://chromiumcodereview.appspot.com/10440030

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@138886 0039d316-1c4b-4281-b951-d872f2087c98