Thanks Eric.  I was thrown by this document: http://wiki.python.org/moin/Git which describes fetching the sources from SVN using git.  I'm comfortable doing either, but it doesn't resolve my confusion.

The version of argparse in the python checkout is 1.1: http://hg.python.org/cpython/file/default/Lib/argparse.py
64 __version__ = '1.1' but differs from the SVN version.

whereas the argparse version available via google code is 1.2.  The diffs indicate several changes not related to the change I'm attempting to make, which prevent my patch from applying cleanly.  Looks like the HG version includes the 1.2 code... but I'm not sure why it would differ from SVN's trunk.