Merge branch 'ps/fsck-stream-from-the-right-object-instance'

"fsck" iterates over packfiles and its access to pack data caused the list to be permuted, which caused it to loop forever; the code to access pack data by "fsck" has been updated to avoid this. * ps/fsck-stream-from-the-right-object-instance: pack-check: fix verification of large objects packfile: expose function to read object stream for an offset object-file: adapt `stream_object_signature()` to take a stream t/helper: improve "genrandom" test helper