Use gdbpy_ref in py-cmd.c

Use gdbpy_ref in py-cmd.c

This changes py-cmd.c to use gdbpy_ref in more places.  This also
fixes a latent memory leak in cmdpy_completer_helper, which
unnecessarily increfs the result of PyObject_CallMethodObjArgs.  This
is not needed because that function returns a new reference.

2017-01-10  Tom Tromey  <tom@tromey.com>

	* python/py-cmd.c (cmdpy_completer_helper): Use gdbpy_ref.  Remove
	extra incref.
	(cmdpy_completer_handle_brkchars, cmdpy_completer, cmdpy_init):
	Use gdbpy_ref.