pgsql: Fix vac_update_relstats to ensure it always sends a relcache

Log Message:
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Fix vac_update_relstats to ensure it always sends a relcache inval message,
even if none of the fields in the pg_class row change.  This behavior is
necessary to ensure other backends flush rd_targblock values that might
point to truncated-away pages.  We got this right pre-8.2 but it was broken
by overoptimistic change to not write out the pg_class row if unchanged.
Per report from Pavan Deolasee.

Tags:
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REL8_2_STABLE

Modified Files:
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    pgsql/src/backend/commands:
        vacuum.c (r1.342 -> r1.342.2.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuum.c.diff?r1=1.342&r2=1.342.2.1)