Fix `tmux` sessions
This will create a new session named `topgrade`, `topgrade-1`,
`topgrade-2`, using the first nonexistent session name it finds. That
session will have a window in it named `topgrade` in which `topgrade` is
run. If `topgrade --tmux` is being run from within tmux, it won't attach
to the new tmux session. If the user is not currently in tmux, it will
attach to the newly-created session.
Co-authored-by: Thomas Schönauer <37108907+DottoDev@users.noreply.github.com>
