Update nano editor to version 4.9.1

 2020.03.31 - GNU nano 4.9.1 "Sapperdeflap"
 Two bugs introduced in version 4.9 are fixed: the cursor
 getting misplaced when undoing line cuts, and filtering
 of the whole buffer to a new buffer not working.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>