midx: the fanout table entries can be 4 bytes, not 8.

midx: the fanout table entries can be 4 bytes, not 8.

I was trying to be future-proof, but it was kind of overkill, since a 32-bit
fanout entry could handle a total of 4 billion *hashes* per midx.  That
would be 20*4bil = 80 gigs in a single midx.  This corresponds to about 10
terabytes of packs, which isn't inconceivable... but if it happens, you
could just use more than one midx.  Plus you'd likely run into other weird
bup problems before your midx files get anywhere near 80 gigs.