The "-u" flag means "update the working tree files", but to
other types of merges, it also implies "I want to keep my local
changes" -- because they prevent local changes from getting lost
by using verify_uptodate.  The one-way merge is different from
other merges in that its purpose is opposite of doing something
else while keeping unrelated local changes.  The point of
one-way merge is to nuke local changes.  So while it feels
somewhat wrong that this actively loses local changes, it is the
right thing to do.
Signed-off-by: NJunio C Hamano <junkio@cox.net>