diffhelpers: use Py_ssize_t in _fix_newline()
Eliminates
mercurial/diffhelpers.c(23) : warning C4244: 'initializing' : conversion from
'Py_ssize_t' to 'int', possible loss of data
mercurial/diffhelpers.c(26) : warning C4244: 'initializing' : conversion from
'Py_ssize_t' to 'int', possible loss of data
mercurial/diffhelpers.c(27) : warning C4244: 'initializing' : conversion from
'Py_ssize_t' to 'int', possible loss of data
mercurial/diffhelpers.c(30) : warning C4244: 'initializing' : conversion from
'Py_ssize_t' to 'int', possible loss of data
when compiling for Windows x64 target using the Microsoft compiler.