message: Fix buggy selection when scrolling down
  When scrolling down with a selection on screen the first line was not
  properly selected. This has been fixed by making sure the selection
  always starts in the first cell when it is only partially visible.
diff: @@ -134,7 +134,7 @@ impl<'a> RenderableCellsIter<'a> {
  Some((start_line, loc.start.col, end_line, loc.end.col))
  },
  (Some(start_line), None) => {
  - Some((start_line, loc.start.col, Line(0), grid.num_cols()))
  + Some((start_line, loc.start.col, Line(0), Column(0)))
  },
  (None, Some(end_line)) => {
  Some((grid.num_lines(), Column(0), end_line, loc.end.col))
  