The `kdiff3 --auto` help message is, "No GUI if all conflicts are auto-
solvable."  This flag was carried over from the original mergetool
commands.  diff_cmd() is for two-way comparisons only so remove the
superfluous flag.
Signed-off-by: NDavid Aguilar <davvid@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>