Merge branch 'po/read-graft-line'

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