Fix bugs in new indexing code.

Fix bugs in new indexing code.

The logic was way too screwy, so I've simplified it a lot.  Also extended
the unit tests quite a bit to replicate the weird problems I was having.  It
seems pretty stable - and pretty fast - now.

Iterating through an index of my whole home directory (bup index -p ~) now
takes about 5.1 seconds, vs. 3.5 seconds before the rewrite.  However,
iterating through just a *fraction* of the index can now bypass all the
parts we don't care about, so it's much much faster than before.

Could probably still stand some more optimization eventually, but at least
the file format allows for speed.  The rest is just code :)