patch 8.1.0360: using an external diff program is slow and inflexible

Problem:    Using an external diff program is slow and inflexible.
Solution:   Include the xdiff library. (Christian Brabandt, closes #2732)
            Use it by default.

src/xdiff/xinclude.h

@@ -0,0 +1,61 @@
+/*
+ *  LibXDiff by Davide Libenzi ( File Differential Library )
+ *  Copyright (C) 2003  Davide Libenzi
+ *
+ *  This library is free software; you can redistribute it and/or
+ *  modify it under the terms of the GNU Lesser General Public
+ *  License as published by the Free Software Foundation; either
+ *  version 2.1 of the License, or (at your option) any later version.
+ *
+ *  This library is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ *  Lesser General Public License for more details.
+ *
+ *  You should have received a copy of the GNU Lesser General Public
+ *  License along with this library; if not, see
+ *  <http://www.gnu.org/licenses/>.
+ *
+ *  Davide Libenzi <davidel@xmailserver.org>
+ *
+ */
+
+/* defines HAVE_ATTRIBUTE_UNUSED */
+#ifdef HAVE_CONFIG_H
+# include "../auto/config.h"
+#endif
+
+/* Mark unused function arguments with UNUSED, so that gcc -Wunused-parameter
+ * can be used to check for mistakes. */
+#ifdef HAVE_ATTRIBUTE_UNUSED
+# define UNUSED __attribute__((unused))
+#else
+# define UNUSED
+#endif
+
+#if defined(_MSC_VER)
+# define inline __inline
+#endif
+
+#if !defined(XINCLUDE_H)
+#define XINCLUDE_H
+
+#include <ctype.h>
+#include <stdio.h>
+#include <stdlib.h>
+#if !defined(_WIN32)
+#include <unistd.h>
+#endif
+#include <string.h>
+#include <limits.h>
+
+#include "xmacros.h"
+#include "xdiff.h"
+#include "xtypes.h"
+#include "xutils.h"
+#include "xprepare.h"
+#include "xdiffi.h"
+#include "xemit.h"
+
+
+#endif /* #if !defined(XINCLUDE_H) */