This speeds up the following scenario:

1) type =B1 in A1. Leave A2 empty.
2) Select A1:A2 and Ctrl-C to copy.
3) Select A3:A50000 (or longer if you so wish), and ctrl-V to paste.

This causes the broadcaster storage array in column B to be heavily
partitioned due to the empty cells interspersed between formula cells
in column A.  Without tracking the column position this would cause a
O(n^2) complexity algorithm.

Change-Id: Ic2f23c2c2bea3353c517faa73fe5412c7528bd95