fix building wrong PROMPT_COMMAND

If one previously declared a perfectly valid `PROMPT_COMMAND` like `PROMPT_COMMAND="do_something;"`, *byobu* would have corrupted it: `PROMPT_COMMAND="$PROMPT_COMMAND;history -a;history -r"` resulted in `"do_something;;history -a;history -r"`, and the double semicolons are illegal: `bash: PROMPT_COMMAND: line 0: syntax error near unexpected token `;'`