Fix diff header comments and missing const

Based on the recent work to wrap diff in objective-git, this
includes a fix for a missing const and a number of clarifications
of the documentation.

Version info for public structs

Hey that's not an int!

status should ignore conflicts entries in the index

odb-pack: resurrect pack_backend__read_header

EINVALIDSPEC

To paraphrase @peff:

You can get both size and type from a packed object reasonably cheaply.
If you have:

* An object that is not a delta; both type and size are available in the
  packfile header.
* An object that is a delta. The packfile type will be OBJ_*_DELTA, and
  you have to resolve back to the base to find the real type. That means
  potentially a lot of packfile index lookups, but each one is
  relatively cheap. For the size, you inflate the first few bytes of the
  delta, whose header will tell you the resulting size of applying the
  delta to the base.

For simplicity, we just decompress the whole delta for now.