diff3: Use logical-not, not bitwise for booleans

And compare impcompat != 0 as it's actually an integer incremented on
each use of one of the AeExX3 options.

Reviewed by: fuz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D53808

(cherry picked from commit abcb80f46c2607fc16564ca87cc25f0908f29f99)
(cherry picked from commit 56bdfd2d967ac104dc599bced168e1a200aa168e)