changegroupv4: add sidedata helpers
These helpers carry the information and computers needed to rewrite sidedata
when generating/applying patches. We will be making use of them soon.
Differential Revision: https://phab.mercurial-scm.org/D10029
#include <Python.h>
#if PY_MAJOR_VERSION >= 3
#define PYCODETYPE PyObject
#else
#define PYCODETYPE PyCodeObject
#endif
namespace contrib
{
void initpy(const char *cselfpath);
PyObject *pyglobals();
} /* namespace contrib */