pgsql: Avoid marking buffer dirty when VACUUM has no work to do.

Avoid marking buffer dirty when VACUUM has no work to do.
When wal_level = 'hot_standby' we touched the last page of the
relation during a VACUUM, even if nothing else had happened.
That would alter the LSN of the last block and set the mtime
of the relation file unnecessarily. Noted by Thom Brown.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/c1458cc495ff800cd176a1c2e56d8b62680d9b71

Modified Files
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src/backend/access/nbtree/nbtpage.c |   12 ++++++++----
1 files changed, 8 insertions(+), 4 deletions(-)