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

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

"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