mitigate tmux displaying its "<... not ready>" message, closes #496

the "flickering" perceived when the "<... not ready>" message is long won't be
totally eliminated until tmux 3.3 though (commit 38c5788232e0e3abdd08ade55a9d4fbcda637df1)

.tmux.conf

@@ -558,6 +558,7 @@ run 'cut -c3- ~/.tmux.conf | sh -s _apply_configuration'
 # }
 #
 # _username() {
+#   echo
 #   tty=${1:-$(tmux display -p '#{pane_tty}')}
 #   ssh_only=$2
 #
@@ -582,6 +583,7 @@ run 'cut -c3- ~/.tmux.conf | sh -s _apply_configuration'
 # }
 #
 # _hostname() {
+#   echo
 #   tty=${1:-$(tmux display -p '#{pane_tty}')}
 #   ssh_only=$2
 #   full=$3
@@ -618,6 +620,7 @@ run 'cut -c3- ~/.tmux.conf | sh -s _apply_configuration'
 # }
 #
 # _root() {
+#   echo
 #   tty=${1:-$(tmux display -p '#{pane_tty}')}
 #   root=$2
 #