object access API
=================
Talk about <sha1_file.c> and <object.h> family, things like
* read_sha1_file()
* read_object_with_reference()
* has_sha1_file()
* write_sha1_file()
* pretend_sha1_file()
* lookup_{object,commit,tag,blob,tree}
* parse_{object,commit,tag,blob,tree}
* Use of object flags
(JC, Shawn, Daniel, Dscho, Linus)