Simplify Tmux auto-joining

This seems to have changed quite a bit since I was using this regularly
(~8 years ago) and in my testing today seems to create a situation where
every separate session is bound to the same active window. So, while it
created a new tmux window (tab) each time I connected, it unfortunately
kept all my Terminal windows/sessions in sync so I couldn't view two
different Tmux windows within the same session using two different
terminal windows.

shells/tmux.sh

@@ -1,19 +1,12 @@
-# I always want to be in a Tmux session. Always.
-#
-# This creates a single always-running "login" session, and creates
-# new session as needed that are bound to "login"'s window group.
-# This lets me have a different terminals tabs/windows have the same tmux
-# windows but be looking at different ones individually
+# I always want to be in a Tmux session when SSHing into a box.
 
-# If we aren't in Tmux or emacs, set it up
+# If we aren't already in Tmux or emacs, set it up
 if command -v tmux &>/dev/null && [ "$SSH_CONNECTION" -a -z "$TMUX" -a -z "$INSIDE_EMACS" -a -z "$EMACS" -a -z "$VIM" -a -z "$VIMRUNTIME" ]; then
     if tty >/dev/null; then
-        if [ -z "$(tmux ls | grep 'login:')" ]; then
-            tmux new-session -d -s login # Create a detached session called login
-            tmux new-session -t login    # Create a *new* session bound to the same windows
+        if ! tmux has-session -t login 2>/dev/null; then
+            tmux attach-session -t login
         else
-            last_session="$(tmux list-windows -t login | tail -n1 | cut -d: -f1)"
-            tmux new-session -t login \; new-window -a -t $last_session # Create a *new* session bound to "login" and create a new window
+            tmux new-session -s login
         fi
 
         # When Tmux exits, we exit