sha1_object_info_extended: make type calculation optional

sha1_object_info_extended: make type calculation optional

Each caller of sha1_object_info_extended sets up an
object_info struct to tell the function which elements of
the object it wants to get. Until now, getting the type of
the object has always been required (and it is returned via
the return type rather than a pointer in object_info).

This can involve actually opening a loose object file to
determine its type, or following delta chains to determine a
packed file's base type. These effects produce a measurable
slow-down when doing a "cat-file --batch-check" that does
not include %(objecttype).

This patch adds a "typep" query to struct object_info, so
that it can be optionally queried just like size and
disk_size. As a result, the return type of the function is
no longer the object type, but rather 0/-1 for success/error.

As there are only three callers total, we just fix up each
caller rather than keep a compatibility wrapper:

  1. The simpler sha1_object_info wrapper continues to
     always ask for and return the type field.

  2. The istream_source function wants to know the type, and
     so always asks for it.

  3. The cat-file batch code asks for the type only when
     %(objecttype) is part of the format string.

On linux.git, the best-of-five for running:

  $ git rev-list --objects --all >objects
  $ time git cat-file --batch-check='%(objectsize:disk)'

on a fully packed repository goes from:

  real    0m8.680s
  user    0m8.160s
  sys     0m0.512s

to:

  real    0m7.205s
  user    0m6.580s
  sys     0m0.608s

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>