Change the behavior of specifying --merged & --no-merged to be an
error, instead of silently picking the option that was provided last.
Subsequent changes of mine add a --no-contains option in addition to
the existing --contains. Providing both of those isn't an error, and
has actual meaning.
Making its cousins have different behavior in this regard would be
confusing to the user, especially since we'd be silently disregarding
some of their command-line input.
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>