`copy_exec` was stupid since it brought unnecessary shell overhead even
though it was meant to improve performance (one could argue that it is
not even necessary here).
Continue the idea of reducing calls to `tmux` by "globally" chaining
together all copy-mode commands instead of doing it only "locally" with
`copy_exec`. This has the added benefit that also calls having arguments
can be included and merged.
scroll_region_{upper,lower} are only modified by the DECSTBM escape
sequence:
> This control function sets the top and bottom margins for the current
> page. You cannot perform scrolling outside the margins.
And are 0 and pane_height - 1 otherwise. Tmux implements these such that
command output can't be displayed outside these margins, but the cursor
in copy-mode can. This means that using them to check if the cursor is
at the very top or bottom can break in certain situation (even those
seem extremely rare since I have never heard of this escape sequence).
Get rid of them and just use 0 and the pane height.
[1]: https://vt100.net/docs/vt510-rm/DECSTBM.html
