filemerge: use leverage `util.readfile()` in `_maketempfiles()`
Differential Revision: https://phab.mercurial-scm.org/D12191
#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 */