pgsql-server: RecentXmin is too recent to use as the cutoff point for

Log Message:
-----------
RecentXmin is too recent to use as the cutoff point for accessing
pg_subtrans --- what we need is the oldest xmin of any snapshot in use
in the current top transaction.  Introduce a new variable TransactionXmin
to play this role.  Fixes intermittent regression failure reported by
Neil Conway.

Modified Files:
--------------
    pgsql-server/src/backend/access/transam:
        subtrans.c (r1.5 -> r1.6)

(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/access/transam/subtrans.c.diff?r1=1.5&r2=1.6)
        transam.c (r1.61 -> r1.62)

(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/access/transam/transam.c.diff?r1=1.61&r2=1.62)
    pgsql-server/src/backend/storage/ipc:
        sinval.c (r1.73 -> r1.74)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/storage/ipc/sinval.c.diff?r1=1.73&r2=1.74)
    pgsql-server/src/backend/utils/time:
        tqual.c (r1.78 -> r1.79)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/utils/time/tqual.c.diff?r1=1.78&r2=1.79)
    pgsql-server/src/include/utils:
        tqual.h (r1.52 -> r1.53)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/utils/tqual.h.diff?r1=1.52&r2=1.53)