Make deduplication use number of key attributes.

Make deduplication use number of key attributes.

Use IndexRelationGetNumberOfKeyAttributes() rather than
IndexRelationGetNumberOfAttributes() when determining whether or not two
index tuples are suitable for merging together into a single posting
list tuple.  This is a little bit tidier.  It brings affected code in
nbtdedup.c a little closer to similar, related code in nbtsplitloc.c.