PackIdxList.refresh(): remember to exclude old midx files.

PackIdxList.refresh(): remember to exclude old midx files.

Previously, if you called refresh(), it would fail to consider
the contents of already-loaded .midx files as already-loaded.  That means
it would load all the constituent .idx files, so you'd actually lose all the
advantages of the .midx after the first refresh().

Thus, the midx optimization mainly worked before you filled up your first
pack (about 1GB of data saved) or until you got an index suggestion.  This
explains why backups would slow down significantly after running for a
while.

Also, get rid of the stupid forget_packs option; just automatically prune
the packs that aren't relevant after the refresh.  This avoids the
possibility of weird behaviour if you set forget_packs incorrectly (which we
did).