terminal_block ->remove: Take the command out of the buffer altogether: the lines it covers go
and the text under them moves up into the gap. The block is dropped from terminal_block<-blocks
with its terminal_block<-terminal
set to @nil.
There is no undo.
The buffer gets shorter at the end the client is writing to, so the
older history is left where it is. Everything that names a line by its
place in the scrollback is moved with the text: the caret, the window
and the marks of every other command. The selection and a running
incremental search are ended rather than followed, since what they were
on may be what is going.
Fails for a command still running, for one whose output has not been
marked as ended, and while an application holds the alternate screen.
