The emacs bug is that isearch cannot search through invisible text
when the 'invisible' property is a list.
The patch adds `notmuch-isearch-range-invisible' function which
is the same as `isearch-range-invisible' but with fixed Emacs bug
#8721. Advice added for `isearch-range-invisible' which calls
`notmuch-isearch-range-invisible' instead of the original
`isearch-range-invisible' when in `notmuch-show-mode'.