Fix mergemaster -U by forcing FreeBSD 9 compatiblity in mtree when mtree is
nmtree.

The mtree output used by mergemaster in this case was clearly not meant for
computer consumption and an approach based on -f <file1> -f <file2> would
probalby be a better idea, but this is a minimal change.

MFC after:	3 days
X-MFC-with:	r258437