bdiff: use ssize_t in favor of Py_ssize_t in cpython-unaware locations
This function and struct will be exposed via cffi, so we need to
remove the cpython API dependency they currently have.
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a