Since commit dac0aa1d, nano would leave edittop at current after reading
data from standard input, meaning that everything that had been read in
was invisible, "scrolled off" the top of the screen.  Correct this by
explicitly setting edittop.

The above does not happen for ^R (^X) because it has a special mechanism
to save and restore edittop.  Nor does it happen for ^R ^X M-F because it
sets current to the top of the file and refreshing the screen will make
edittop follow, or -- in case just one unterminated line was read in --
edittop will already be equal to current.