cmd-midx: correctly handle a tiny nonzero number of objects.

cmd-midx: correctly handle a tiny nonzero number of objects.

If all the sha1sums would have fit in a single page, the number of bits in
the table would be negative, with odd results.  Now we just refuse to create
the midx if there are too few objects *and* too few files, since it would be
useless anyway.

We're still willing to create a very small midx if it allows us to merge
several indexes into one, however small, since that will still speed up
searching.