Include the existing window and session name in the prompt when renaming

and add a new key binding ($) for rename session. From Tiago Cunha.

cmd-command-prompt.c

@@ -59,8 +59,13 @@ void
 cmd_command_prompt_key_binding(struct cmd *self, int key)
 {
 	switch (key) {
+	case '$':
+		self->args = args_create(1, "rename-session '%%'");
+		args_set(self->args, 'I', "#S");
+		break;
 	case ',':
 		self->args = args_create(1, "rename-window '%%'");
+		args_set(self->args, 'I', "#W");
 		break;
 	case '.':
 		self->args = args_create(1, "move-window -t '%%'");

key-bindings.c

@@ -106,6 +106,7 @@ key_bindings_init(void)
 		{ '!', 			  0, &cmd_break_pane_entry },
 		{ '"', 			  0, &cmd_split_window_entry },
 		{ '#', 			  0, &cmd_list_buffers_entry },
+		{ '$',			  0, &cmd_command_prompt_entry },
 		{ '%', 			  0, &cmd_split_window_entry },
 		{ '&', 			  0, &cmd_confirm_before_entry },
 		{ '(',                    0, &cmd_switch_client_entry },

tmux.1

@@ -233,6 +233,8 @@ Break the current pane out of the window.
 Split the current pane into two, top and bottom.
 .It #
 List all paste buffers.
+.It $
+Rename the current session.
 .It %
 Split the current pane into two, left and right.
 .It &