request explicit return from subprocess

Previously we were recording output from subprocesses using a multiwriter
and hooking that up to the cmd's stdout to write to both os.Stdout and
a buffer. We would then display the output after the program finished.

This worked well for commands like 'ls' but not for commands like 'vi'
which expect you to be in a tty, and when you've got the cmd's stdout
pointing at a multiwriter, the subprogram thinks we're not in a tty
and then things like terminal corruption can happen. This was the case
with neovim, and even in vim a warning was given with a pause before
starting the program.

Now we're chucking out the multiwriter and instead making it that you
need to press enter after the program has finished to return to lazygit.
This allows you to view the output of the program (e.g. if it's ls) and
then decide that you want to return. It's one level of unnecessary
redirection for editors like vim, but even they could potentially have
output to stderr/stdout that you want to look at before returning.

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