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.

mercurial/bdiff.c

@@ -15,12 +15,13 b''
 #include <string.h>
 #include <limits.h>
 
+#include "compat.h"
 #include "util.h"
 #include "bitmanipulation.h"
 
 struct line {
 	int hash, n, e;
-	Py_ssize_t len;
+	ssize_t len;
 	const char *l;
 };
 
@@ -34,7 +35,7 b' struct hunk {'
 	struct hunk *next;
 };
 
-static int splitlines(const char *a, Py_ssize_t len, struct line **lr)
+static int splitlines(const char *a, ssize_t len, struct line **lr)
 {
 	unsigned hash;
 	int i;