emacs: `notmuch-show-get-message-id': optionally return Message-Id sans prefix

* emacs/notmuch-show.el

  (notmuch-show-get-message-id):
    Add optional arg BARE.  When non-nil, return a Message-Id without
    quotes and prefix, thus obviating the need to strip them off again
    in various places.

  (notmuch-show-stash-message-id-stripped):
    Update wrt changes in `notmuch-show-get-message-id'.