pgsql: Repair VACUUM FULL bug introduced by HOT patch: the original way

Log Message:
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Repair VACUUM FULL bug introduced by HOT patch: the original way of
calculating a page's initial free space was fine, and should not have been
"improved" by letting PageGetHeapFreeSpace do it. VACUUM FULL is going to
reclaim LP_DEAD line pointers later, so there is no need for a guard
against the page being too full of line pointers, and having one risks
rejecting pages that are perfectly good move destinations.

This also exposed a second bug, which is that the empty_end_pages logic
assumed that any page with no live tuples would get entered into the
fraged_pages list automatically (by virtue of having more free space than
the threshold in the do_frag calculation). This assumption certainly
seems risky when a low fillfactor has been chosen, and even without
tunable fillfactor I think it could conceivably fail on a page with many
unused line pointers. So fix the code to force do_frag true when notup
is true, and patch this part of the fix all the way back.

Per report from Tomas Szepe.

Tags:
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REL8_2_STABLE

Modified Files:
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pgsql/src/backend/commands:
vacuum.c (r1.342.2.4 -> r1.342.2.5)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuum.c?r1=1.342.2.4&r2=1.342.2.5)