pgsql: Make ginbuild's funcCtx be independent of its tmpCtx.

Make ginbuild's funcCtx be independent of its tmpCtx.

Previously the funcCtx was a child of the tmpCtx, but that was broken
by commit eaa5808e8ec4e82ce1a87103a6b6f687666e4e4c, which made
MemoryContextReset() delete, not reset, child contexts.  The behavior of
having a tmpCtx reset also clear the other context seems rather dubious
anyway, so let's just disentangle them.  Per report from Erik Rijkers.

In passing, fix badly-inaccurate comments about these contexts.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/1601830ec20d56dc7bf6b60a34f69841429e4825

Modified Files
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src/backend/access/gin/gininsert.c |   11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)