[RFC/PATCH 0/4] cat-file --batch-disk-sizes

When I work with alternates repositories that have the objects for many individual forks inter-mixed, one of the questions I want to ask git is how much space particular forks are taking up in the object database. This is easy enough to script with `rev-list --objects $fork1 --not $fork2`, as long as you can convert the object names into their on-disk sizes.

Unfortunately, it's hard to get the on-disk object sizes for packs. You can do it directly with `verify-pack -v`, which is incredibly slow. Or you can sort and subtract offsets from the output of `show-index` (i.e., the same thing the pack-revindex code does internally). Instead, this patch series exposes the revindex-generated sizes on the command line.

The fourth patch does not need to be built on top of this series, but the early parts provide a convenient way to measure the revindex code.

  [1/4]: zero-initialize object_info structs
  [2/4]: teach sha1_object_info_extended a "disk_size" query
  [3/4]: cat-file: add --batch-disk-sizes option
  [4/4]: pack-revindex: radix-sort the revindex

-Peff