diff3: Code cleanup

• Order includes.

• Turn the diff type into an enum.

• Turn an unreachable error message into an assertion.

• Remove unused debugging code.

• Remove some dead assignments.

• Consistently use 1 instead of EXIT_FAILURE.

• Turn a return from main() into an exit().

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D55517

(cherry picked from commit c2dee7786bf32cb66cedec226e42e79e06457c51)