Refactor nbtree high key truncation.

 Refactor nbtree high key truncation.
 
 Simplify _bt_truncate(), the routine that generates truncated leaf page
 high keys.  Remove a micro-optimization that avoided a second palloc0()
 call (this was used when a heap TID was needed in the final pivot tuple,
 though only when the index happened to not be an INCLUDE index).
 
 Removing this dubious micro-optimization allows _bt_truncate() to use
 the index_truncate_tuple() indextuple.c utility routine in all cases.
 This was already the common case.
 
 This commit is a HEAD-only follow up to bugfix commit 4b42a899.