Factor out message buffer mgmt from notmuch-show-view-all-mime-parts

The ability to temporarily create a buffer containing only the
contents of the currently selected message in notmuch show mode is
generally useful.  This commit factors the majority of the code
required to do so out of notmuch-show-view-all-mime-parts into a macro
called with-current-notmuch-show-message and rewrites the original
function in terms of the macro.

A future set of commits will provide additional functionality using
the macro as well.