pgsql: Fix an additional set of problems in GIN's handling of lossy page

Log Message:
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Fix an additional set of problems in GIN's handling of lossy page pointers.
Although the key-combining code claimed to work correctly if its input
contained both lossy and exact pointers for a single page in a single TID
stream, in fact this did not work, and could not work without pretty
fundamental redesign.  Modify keyGetItem so that it will not return such a
stream, by handling lossy-pointer cases a bit more explicitly than we did
before.

Per followup investigation of a gripe from Artur Dabrowski.
An example of a query that failed given his data set is
select count(*) from search_tab where
(to_tsvector('german', keywords ) @@ to_tsquery('german', 'ee:* | dd:*')) and
(to_tsvector('german', keywords ) @@ to_tsquery('german', 'aa:*'));

Back-patch to 8.4 where the lossy pointer code was introduced.

Tags:
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REL8_4_STABLE

Modified Files:
--------------
    pgsql/src/backend/access/gin:
        ginget.c (r1.27.2.2 -> r1.27.2.3)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gin/ginget.c?r1=1.27.2.2&r2=1.27.2.3)
    pgsql/src/include/access:
        gin.h (r1.34.2.2 -> r1.34.2.3)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/access/gin.h?r1=1.34.2.2&r2=1.34.2.3)