get_delta_base_oid(): convert to new revindex API

Replace direct accesses to the 'struct revindex' type with a call to
'pack_pos_to_index()'.

Likewise drop the old-style 'find_pack_revindex()' with its replacement
'offset_to_pack_pos()' (while continuing to perform the same error
checking).

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

packfile.c

@@ -1235,18 +1235,18 @@ static int get_delta_base_oid(struct packed_git *p,
 		oidread(oid, base);
 		return 0;
 	} else if (type == OBJ_OFS_DELTA) {
-		struct revindex_entry *revidx;
+		uint32_t base_pos;
 		off_t base_offset = get_delta_base(p, w_curs, &curpos,
 						   type, delta_obj_offset);
 
 		if (!base_offset)
 			return -1;
 
-		revidx = find_pack_revindex(p, base_offset);
-		if (!revidx)
+		if (offset_to_pack_pos(p, base_offset, &base_pos) < 0)
 			return -1;
 
-		return nth_packed_object_id(oid, p, revidx->nr);
+		return nth_packed_object_id(oid, p,
+					    pack_pos_to_index(p, base_pos));
 	} else
 		return -1;
 }