emacs: Improve the display of truncated authors.

emacs: Improve the display of truncated authors.

Incremental search does not match strings that span a
visible/invisible boundary. This results in failure to correctly
isearch for authors in `notmuch-search' mode if the name of the author
is split between the visible and invisible components of the authors
string. To avoid this, attempt to truncate the visible component of
the authors string on a boundary between authors, such that the
entirety of an author's name is either visible or invisible.