Fix the last regression shown by cvs2svn

We need to hold a reference to the memoized object in the Pickler for as long as
the Pickler lives.

With this applied, all of cvs2svn's tests that pass under mainline 2.6.1 pass
under Unladen Swallow.

[Collin Winter]

PTAL

[Jeffrey Yasskin]

LGTM

http://codereview.appspot.com/91044/diff/1/2
File Modules/_testcapimodule.c (right):

http://codereview.appspot.com/91044/diff/1/2#newcode834
Line 834: Py_DECREF(three);  /* The memo table takes its own reference. */
This comment seems unnecessary, since incref'ing on insertion is pretty common
in Python. On the other hand, you should have a test that checks that the
objects aren't deallocated while they're in the memotable.

http://codereview.appspot.com/91044/diff/1/3
File Modules/cPickle.c (right):

http://codereview.appspot.com/91044/diff/1/3#newcode382
Line 382: Py_ssize_t i;
Just call PyMemoTable_Clear()? The extra memset shouldn't cost that much since
everything was just decref'ed, and it might even be optimized away.