In the current scrollback PR the `reset` command does not affect the
scrollback history. To make sure the terminal is properly reset, it
should clear the scrollback history.
This commit fixes this by creating a new and empty grid whenever `reset`
is executed. It takes the current dimensions and history size from the
old grid.
Right now there's an empty ref-test called `grid_reset` without any
content, this should be implemented once #1244 is resolved.
This fixes#1242.
Previously ref-tests just ignored the scrollback history to keep
the old tests working, this would lead to new tests which rely on
scrollback history to succeeed even though they should not.
This has been fixed and it is now possible to create ref-tests with and
without scrollback history. When available the scrollback history is
compared, but the old tests still work without having to adjust them.
This fixes#1244.
