Conversion from uchar[20] to struct object_id continues; this is to
ensure that we do not assume sizeof(struct object_id) is the same
as the length of SHA-1 hash (or length of longest hash we support).
* po/read-graft-line:
commit: rewrite read_graft_line
commit: allocate array using object_id size
commit: replace the raw buffer with strbuf in read_graft_line
sha1_file: fix definition of null_sha1