Abbreviated keys for Datum tuplesort (was: Re: B-Tree support function number 3 (strxfrm() optimization))

[pruning the Cc: list and starting a new thread]

Here's the cleaned-up version of the patch to allow abbreviated keys
when sorting a single Datum. This also removes comments that suggest
that the caller of tuplesort_begin_datum should ever have to care
about the abbreviated key optimization.

I'll add this to the CF.

--
Andrew (irc:RhodiumToad)