Add a "back to indentation" key in copy mode to move the cursor to the first

non-whitespace character. ^ with vi and M-m with emacs key bindings. Another
from Kalle Olavi Niemitalo, thanks.

tmux.h

@@ -357,6 +357,7 @@ struct msg_resize_data {
 /* Editing keys. */
 enum mode_key_cmd {
 	MODEKEYCMD_BACKSPACE = 0x1000,
+	MODEKEYCMD_BACKTOINDENTATION,
 	MODEKEYCMD_CHOOSE,
 	MODEKEYCMD_CLEARSELECTION,
 	MODEKEYCMD_COMPLETE,