Use tmux window option for pane change detection

Improve tmux integration when changing buffers or panes when vim was
invoked by another process so that seamless navigation works when the
`vim$TMUX_PANE` window option has been set, not only when the pane's
command matches a simple regex.

plugin/autocmds.vim

@@ -1,6 +1,13 @@
 augroup benieAugroup
   autocmd!
 
+  if $TMUX !=# ''
+    " Set tmux window option to detect when to change pane.
+    let s:tmux_option = '@vim'.substitute($TMUX_PANE, '%', '\\%', 'g')
+    call system('tmux set-window-option '.s:tmux_option.' 1')
+    au VimLeave * call system('tmux set-window-option -u '.s:tmux_option)
+  endif
+
   " Reopening a file at last curson position
   au BufReadPost * if line("'\"") > 0 && line("'\"") <= line("$")
           \ | exe "normal! g'\"" | endif