master/graph: fix cdef expansion to handle repeated field names

This fixes handling of a cdef such as
 hit.cdef hit,hit,miss,+,/
which computes hit rate hit/(hit+miss) from hits and misses.
Without this patch, only the first hit is expanded as substitutions overlap. The fix was to use look-aheads (which are not part or substitutions). After this the substitutions do not overlap any more.