merge-ort: mark conflict/warning messages from inner merges as omittable

merge-ort: mark conflict/warning messages from inner merges as omittable

A recursive merge involves merging the merge bases of the two branches
being merged.  Such an inner merge can itself generate conflict notices.
While such notices may be useful when initially trying to create a
merge, they seem to just be noise when investigating merges later with
--remerge-diff.  (Especially when both sides of the outer merge resolved
the conflict the same way leading to no overall conflict.)  Remove them.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>