streamclone: consider secret changesets (BC) (issue5589) Previously, a repo containing secret changesets would be served via stream clone, transferring those secret changesets. While secret changesets aren't meant to imply strong security (if you really want to keep them secret, others shouldn't have read access to the repo), we should at least make an effort to protect secret changesets when possible. After this commit, we no longer serve stream clones for repos containing secret changesets by default. This is backwards incompatible behavior. In case anyone is relying on the behavior, we provide a config option to opt into the old behavior. Note that this defense is only beneficial for remote repos accessed via the wire protocol: if a client has access to the files backing a repo, they can get to the raw data and see secret revisions.