bdiff: fix yet more fallout from xdiff long/int64 conversion (issue5885)

Authored by jcristau.

Description

bdiff: fix yet more fallout from xdiff long/int64 conversion (issue5885)

"l" in Py_BuildValue's format string means long, so passing int64_t
instead results in fireworks on 32bit architectures.

Differential Revision: https://phab.mercurial-scm.org/D3538