bug#16544: Optimazation for is_mb_middle

[Jim Meyering]

I have just noticed that this (currently topmost) commit lists my name
as the author.
That was not my intention.  Norihiro Tanaka should be listed as the author.
I plan to take the unusual step of replacing that public/pushed commit with an
identical one with that important correction.  Then I will post
instructions telling people
how to deal with this unusual situation.

Norihiro, you can help avoid recurrence by submitting future patches created
by running "git format-patch --stdout -1".  That way, your name is recorded
in the patch, and I don't have to take extra steps to add it when
creating a commit.