emacs: Optionally retain the state of the buffer during `notmuch-show-refresh-view'.

With an argument, record and reply the state of the buffer during
`notmuch-show-refresh-view'.

In this context, "state" is defined as:
 - the open/closed state of each message,
 - the current message.

Traditional use of refresh with the = key does not retain the
state. The recently introduced toggle commands ($, !, < and >) do
retain the state.