xdiff: use ptrdiff_t for dstart/dend

ptrdiff_t is appropriate for dstart and dend because they both describe
positive or negative offsets relative to a pointer.

Signed-off-by: Ezekiel Newren <ezekielnewren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: ezekielnewren

xdiff/xtypes.h

@@ -47,7 +47,7 @@ typedef struct s_xrecord {
 typedef struct s_xdfile {
 	xrecord_t *recs;
 	long nrec;
-	long dstart, dend;
+	ptrdiff_t dstart, dend;
 	bool *changed;
 	long *rindex;
 	long nreff;