use CALLOC_ARRAY

Add and apply a semantic patch for converting code that open-codes
CALLOC_ARRAY to use it instead.  It shortens the code and infers the
element size automatically.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/coccinelle/array.cocci

@@ -88,3 +88,11 @@ expression n;
 @@
 - ptr = xmalloc((n) * sizeof(T));
 + ALLOC_ARRAY(ptr, n);
+
+@@
+type T;
+T *ptr;
+expression n;
+@@
+- ptr = xcalloc(n, \( sizeof(*ptr) \| sizeof(T) \) )
++ CALLOC_ARRAY(ptr, n)