xdiff: move sign comparison warning guard into each file

Allow each file to fix the warnings guarded by the macro separately by
moving the definition from the shared xinclude.h into each file that
needs it.

xmerge.c and xprepare.c do not contain any signed vs. unsigned
comparisons so the definition was not included in these files.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

xdiff/xemit.c

@@ -20,6 +20,8 @@
  *
  */
 
+#define DISABLE_SIGN_COMPARE_WARNINGS
+
 #include "xinclude.h"
 
 static long xdl_get_rec(xdfile_t *xdf, long ri, char const **rec) {

xdiff/xhistogram.c

@@ -41,6 +41,8 @@
  * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#define DISABLE_SIGN_COMPARE_WARNINGS
+
 #include "xinclude.h"
 
 #define MAX_PTR	UINT_MAX

xdiff/xinclude.h

@@ -23,8 +23,6 @@
 #if !defined(XINCLUDE_H)
 #define XINCLUDE_H
 
-#define DISABLE_SIGN_COMPARE_WARNINGS
-
 #include "git-compat-util.h"
 #include "xmacros.h"
 #include "xdiff.h"

xdiff/xpatience.c

@@ -19,6 +19,9 @@
  *  Davide Libenzi <davidel@xmailserver.org>
  *
  */
+
+#define DISABLE_SIGN_COMPARE_WARNINGS
+
 #include "xinclude.h"
 
 /*

xdiff/xutils.c

@@ -20,6 +20,8 @@
  *
  */
 
+#define DISABLE_SIGN_COMPARE_WARNINGS
+
 #include "xinclude.h"