590b045c3 made it so tuplestore.c would store tuples inside a
generation.c memory context. After fixing a bug report in 97651b013, it
seems that it's probably best not to allocate BufFile related
allocations in that context. Let's keep it just for tuple data.
This adjusts the code to switch to the Tuplestorestate.context's parent,
which is the MemoryContext that tuplestore_begin_common() was called in.
It does not seem worth adding a new field in Tuplestorestate to store
this when we can access it by looking at the Tuplestorestate's
context's parent.
Discussion: https://postgr.es/m/CAApHDvqFt_CdJtSr+E9YLZb7jZAyRCy3hjQ+ktM+dcOFVq-xkg@mail.gmail.com
04bcf9e19a