Use a PackBitmap file as a quicker way to check .idx files.

Use a PackBitmap file as a quicker way to check .idx files.

When we receive a new .idx file, we auto-generate a .map file from it.  It's
essentially an allocation bitmap: for each 20-bit prefix, we assign one bit
to tell us if that particular prefix is in that particular packfile.  If it
isn't, there's no point searching the .idx file at all, so we can avoid
mapping in a lot of pages.  If it is, though, we then have to search the
.idx *too*, so we suffer a bit.

On the whole this reduces memory thrashing quite a bit for me, though.
Probably the number of bits needs to be variable in order to work over a
wider range of packfile sizes/numbers.