patch 9.1.1306: completion menu rendering can be improved

patch 9.1.1306: completion menu rendering can be improved

Problem:  Parts of the popup menu were rendered twice when the popup was
          at maximum width because the truncation flag was being set too
          liberally.
Solution: Make the truncation condition more precise by only setting it
          when there's exactly one character of space remaining
          (glepnir).

closes: #17108

Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>