Fix tmux panic (#165)

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>

src/main.rs

@@ -82,7 +82,8 @@ fn run() -> Result<()> {
     if config.run_in_tmux() && env::var("TOPGRADE_INSIDE_TMUX").is_err() {
         #[cfg(unix)]
         {
-            tmux::run_in_tmux(config.tmux_arguments()?);
+            tmux::run_in_tmux(config.tmux_arguments()?)?;
+            return Ok(());
         }
     }