cmd/bloom: map only one .idx file at a time.

cmd/bloom: map only one .idx file at a time.

This massively decreases virtual memory allocation since we only ever need
to look at a single idx at once.

In theory, VM doesn't cost us anything, but on 32-bit systems we can
actually run out of address space if we try to map all the idx files at
once on a very large repo.

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>