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

Log Message:
-----------
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.

Modified Files:
--------------
pgsql/src/backend/access/gin:
ginget.c (r1.31 -> r1.32)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gin/ginget.c?r1=1.31&r2=1.32)
pgsql/src/include/access:
gin.h (r1.40 -> r1.41)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/access/gin.h?r1=1.40&r2=1.41)