pgsql: Don't use SnapshotNow in get_actual_variable_range.

Don't use SnapshotNow in get_actual_variable_range.

Instead, use the active snapshot.  Per Tom Lane, this function is
most interested in knowing the range of tuples our scan will actually
see.

This is another step towards full removal of SnapshotNow.

Branch
------
master

Details
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http://git.postgresql.org/pg/commitdiff/3483f4332de73e038db64be0614219b64fd3c971

Modified Files
--------------
src/backend/utils/adt/selfuncs.c |    9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)