Speed up display of long lines under 'truncate-lines'

Speed up display of long lines under 'truncate-lines'

* src/xdisp.c (partial_line_height): Return zero for long and
truncated lines.
(fast_move_it_horizontally): New function.
(hscroll_window_tree, display_line): Use
'fast_move_it_horizontally' in preference to
'move_it_in_display_line_to', when dealing with long and truncated
lines.
(redisplay_internal): Optimize "optimization 3" for long and
truncated lines.
* src/buffer.c (syms_of_buffer) <large-hscroll-threshold>: New
variable.

* etc/NEWS: Announce 'large-hscroll-threshold'.