Code in slot_getallattrs() is the same as if slot_getsomeattrs() is
called with number of attributes specified in the tuple
descriptor. Implement it that way instead of duplicating the code
between those two functions.
This is part of a patchseries abstracting TupleTableSlots so they can
store arbitrary forms of tuples, but is a nice enough cleanup on its
own.
Author: Ashutosh Bapat
Reviewed-By: Andres Freund
Discussion: https://postgr.es/m/20180220224318.gw4oe5jadhpmcdnm@alap3.anarazel.de
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