bdiff: use Python memory allocator in fixws

bdiff: use Python memory allocator in fixws

Python has its own memory allocation APIs. For allocations
<= 512 bytes, it allocates memory from arenas. This means that
average small allocations don't call the system allocator, which
makes them faster. Also, arena allocations cut down on memory
fragmentation, which can matter for performance in long-running
processes.

Another advantage of using the Python memory allocator is that
allocations are tracked by Python. This is a bigger deal in
Python 3, as modern versions of Python have some decent built-in
tools for examining memory usage, leaks, etc.

This patch converts a trivial malloc() + free() in the bdiff code
to use the Python allocator APIs. Since the object being
operated on is a line, chances are it will use an arena. So,
this could have a net positive impact on performance (although
I didn't measure it).

--- a/mercurial/bdiff_module.c	Thu Mar 16 11:17:55 2017 -0700
+++ b/mercurial/bdiff_module.c	Thu Mar 09 11:54:25 2017 -0800
@@ -158,7 +158,7 @@
 	r = PyBytes_AsString(s);
 	rlen = PyBytes_Size(s);
 
-	w = (char *)malloc(rlen ? rlen : 1);
+	w = (char *)PyMem_Malloc(rlen ? rlen : 1);
 	if (!w)
 		goto nomem;
 
@@ -178,7 +178,7 @@
 	result = PyBytes_FromStringAndSize(w, wlen);
 
 nomem:
-	free(w);
+	PyMem_Free(w);
 	return result ? result : PyErr_NoMemory();
 }