Split up signed references into its read and write components.
On the write side:
- Preserve the old behavior of writing references to the blob `/refs`
and sign over the blob.
- Ensure `refs/rad/root` is contained in the `/refs` blob.
- Ensure `refs/rad/sigrefs` is *not* contained in the `/refs` blob.
- Introduce a new (internal) reference `refs/rad/sigrefs-parent`
so that no two `/refs` blob are equal, even if they contain
the same set of (non-internal) refs.
On the read side:
- Preserve the verification of the signature in `/signature` and
the reference `refs/rad/root` (if present).
- Fail verification of `refs/rad/root` is not present.
- Protect against replay attacks by walking the history of the
head of `refs/rad/sigrefs`, skipping interpretation of `/refs`
blobs in case they are identical to a previous `/refs` blob.
This is achieved by searching for repeated contents of the
`/signature` blob.
The reference `refs/rad/sigrefs-parent` is never read from or written to
the Git repository in storage.
The pre-existing implementation of signed references did not include
a nonce, thus duplicate but legitimate sets of references could not
be distinguished from maliciously replayed sets of references.
The new implementation uses `radicle-git-metadata` which is moved from
`dev-dependencies` to `dependencies`.
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