emacs/tree: add call to notmuch-hl-line-mode from process-filter

emacs/tree: add call to notmuch-hl-line-mode from process-filter

This removes the visual gap/stutter between when the screen fills and
when the hl-line "cursor" is drawn.  It is not obviously how to
robustly test this, since it the observable effect is purely a matter
of timing.