xdiff: change rindex from long to size_t in xdfile_t

rindex describes a index offset which means it's an index into memory
which should use size_t.

Changing the type of rindex from long to size_t has no cascading
refactor impact because it is only ever used to directly index other
arrays.

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

Author: ezekielnewren

xdiff/xtypes.h

@@ -49,7 +49,7 @@ typedef struct s_xdfile {
 	xrecord_t *recs;
 	size_t nrec;
 	bool *changed;
-	long *rindex;
+	size_t *rindex;
 	size_t nreff;
 	ptrdiff_t dstart, dend;
 } xdfile_t;