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)