t / oid-info / hash-infoon commit commit-graph tests: fix unportable "dd" invocation (b9cc405)
   1rawsz sha1:20
   2rawsz sha256:32
   3
   4hexsz sha1:40
   5hexsz sha256:64
   6
   7zero sha1:0000000000000000000000000000000000000000
   8zero sha256:0000000000000000000000000000000000000000000000000000000000000000