pgsql: Fix interaction between materializing holdable cursors and firing

Log Message:
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Fix interaction between materializing holdable cursors and firing
deferred triggers: either one can create more work for the other,
so we have to loop till it's all gone. Per example from andrew(at)supernews(dot)
Add a regression test to help spot trouble in this area in future.

Tags:
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REL8_0_STABLE

Modified Files:
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pgsql/src/backend/access/transam:
xact.c (r1.195 -> r1.195.4.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xact.c.diff?r1=1.195&r2=1.195.4.1)
pgsql/src/backend/commands:
trigger.c (r1.177 -> r1.177.4.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/trigger.c.diff?r1=1.177&r2=1.177.4.1)
pgsql/src/backend/utils/mmgr:
portalmem.c (r1.76.4.1 -> r1.76.4.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mmgr/portalmem.c.diff?r1=1.76.4.1&r2=1.76.4.2)
pgsql/src/include/commands:
trigger.h (r1.51 -> r1.51.4.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/trigger.h.diff?r1=1.51&r2=1.51.4.1)
pgsql/src/include/utils:
portal.h (r1.54 -> r1.54.4.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/portal.h.diff?r1=1.54&r2=1.54.4.1)
pgsql/src/test/regress/expected:
portals.out (r1.8 -> r1.8.4.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/portals.out.diff?r1=1.8&r2=1.8.4.1)
pgsql/src/test/regress/sql:
portals.sql (r1.5 -> r1.5.4.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/portals.sql.diff?r1=1.5&r2=1.5.4.1)