Merge branch 'jk/cleanup-object-parsing-and-fsck' into jch

Crufty code and logic accumulated over time around the object
parsing and low-level object access used in "git fsck" have been
cleaned up.

* jk/cleanup-object-parsing-and-fsck: (23 commits)
  fsck: accept an oid instead of a "struct tree" for fsck_tree()
  fsck: accept an oid instead of a "struct commit" for fsck_commit()
  fsck: accept an oid instead of a "struct tag" for fsck_tag()
  fsck: rename vague "oid" local variables
  fsck: don't require an object struct in verify_headers()
  fsck: don't require an object struct for fsck_ident()
  fsck: drop blob struct from fsck_finish()
  fsck: accept an oid instead of a "struct blob" for fsck_blob()
  fsck: don't require an object struct for report()
  fsck: only require an oid for skiplist functions
  fsck: only provide oid/type in fsck_error callback
  fsck: don't require object structs for display functions
  fsck: use oids rather than objects for object_name API
  fsck_describe_object(): build on our get_object_name() primitive
  fsck: unify object-name code
  fsck: require an actual buffer for non-blobs
  fsck: stop checking tag->tagged
  fsck: stop checking commit->parent counts
  fsck: stop checking commit->tree value
  remember commit/tag parse failures
  ...

diff --cc commit.c
Simple merge