- Check for -h|--help before anything is modified.
- Discard stderr from 'git submodule update'.
- Be careful not to overwrite script while it's executing
  on win.
- Run hooks by invoking 'git runhooks'.
- Fix --dry-run option.

R=iannucci@chromium.org
CC=jar@chromium.org,scottmg@chromium.org

BUG=


Review URL: https://chromiumcodereview.appspot.com/12865010

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@189926 0039d316-1c4b-4281-b951-d872f2087c98