diff3: Produce correct exit status

Use exit status 2 for errors, 1 only to indicate that differences were
found between the inputs (in some operating modes).

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: ngie, bapt
Differential Revision: https://reviews.freebsd.org/D55608

(cherry picked from commit fe5341287c6c9bacc18879b25ed72ceb42e1c811)