Use `without-restriction` in `with-temporary-notmuch-message-buffer`

Use `without-restriction` in `with-temporary-notmuch-message-buffer`

This ensures that the temporary copy of the current message-mode
buffer is whole and not limited by a current restriction.

An example of such restriction is the default one established by
message-mode when composing a reply, that hides the References,
In-Reply-To and similar headers.