tscrollup and tscrolldown do not use tsetdirt, but their code is
equivalent to
tsetdirt(orig, term.bot-n);
tsetdirt(orig+n, term.bot);
tclearregion also marks cleared lines as dirty.
In tscrolldown it sets lines from term.bot-n+1 to term.bot dirty, and in
tscrollup it sets lines from orig to orig+n-1 dirty.
In both functions all lines from orig to term.bot are effectively set
dirty, but in tscrolldown lines from orig+n to term.bot are set dirty
twice, and in tscrollup lines from orig to term.bot-n are set dirty
twice.
These patches make it clear which lines are set dirty and sets them
dirty once in each funciton.
