pgsql: Fix missing rows in query update a=..

Log Message:
-----------
Fix missing rows in query
update a=.. where a... with GiST index on column 'a'
Backpatch from 8.0 branch

Tags:
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REL7_4_STABLE

Modified Files:
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pgsql/src/backend/access/gist:
gist.c (r1.105 -> r1.105.4.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gist/gist.c.diff?r1=1.105&r2=1.105.4.1)
gistget.c (r1.36.4.1 -> r1.36.4.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gist/gistget.c.diff?r1=1.36.4.1&r2=1.36.4.2)
gistscan.c (r1.47 -> r1.47.4.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gist/gistscan.c.diff?r1=1.47&r2=1.47.4.1)