pgsql: Make CLUSTER MVCC-safe.

Log Message:
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Make CLUSTER MVCC-safe. Heikki Linnakangas

Modified Files:
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pgsql/src/backend/access/heap:
Makefile (r1.14 -> r1.15)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/Makefile.diff?r1=1.14&r2=1.15)
heapam.c (r1.231 -> r1.232)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/heapam.c.diff?r1=1.231&r2=1.232)
pgsql/src/backend/access/nbtree:
nbtsort.c (r1.110 -> r1.111)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtsort.c.diff?r1=1.110&r2=1.111)
pgsql/src/backend/commands:
cluster.c (r1.158 -> r1.159)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/cluster.c.diff?r1=1.158&r2=1.159)
tablecmds.c (r1.218 -> r1.219)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablecmds.c.diff?r1=1.218&r2=1.219)
pgsql/src/include/access:
heapam.h (r1.122 -> r1.123)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/heapam.h.diff?r1=1.122&r2=1.123)
pgsql/src/test/regress/expected:
cluster.out (r1.18 -> r1.19)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/cluster.out.diff?r1=1.18&r2=1.19)
pgsql/src/test/regress/sql:
cluster.sql (r1.10 -> r1.11)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/cluster.sql.diff?r1=1.10&r2=1.11)

Added Files:
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pgsql/src/backend/access/heap:
rewriteheap.c (r1.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/rewriteheap.c?rev=1.1&content-type=text/x-cvsweb-markup)
pgsql/src/include/access:
rewriteheap.h (r1.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/rewriteheap.h?rev=1.1&content-type=text/x-cvsweb-markup)