xdiff: use git-compat-util

Since the xdiff library was not originally part of Git, it does its own
system includes. Let's instead use git-compat-util, which has two
benefits:

  1. It adjusts for any system-specific quirks in how or what we should
     include (though xdiff's needs are light enough that this hasn't
     been a problem in the past).

  2. It lets us use wrapper functions like xmalloc().

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

xdiff/xinclude.h

@@ -23,13 +23,7 @@
 #if !defined(XINCLUDE_H)
 #define XINCLUDE_H
 
-#include <ctype.h>
-#include <stdio.h>
-#include <stdlib.h>
-#include <unistd.h>
-#include <string.h>
-#include <limits.h>
-
+#include "git-compat-util.h"
 #include "xmacros.h"
 #include "xdiff.h"
 #include "xtypes.h"