[PROTOCOL UPGRADE] Deduplicate content when creating a revision via signing/witnessing

[rht]

The current limitation with v1.2 is that whenever one signs/witness a revision, a new revision is created which contains the entire duplicate of the previous revision + the signature/witness, which then results in a different verification hash, because the "previous_verification_hash" field has changed too.

There are 4 alternatives to this:

1. Treat every revision metadata as a new element in a linked list that grows one by one whenever an element is added (local_timestamp, witness, signature, etc). The linked list is then secured by a Merkle tree.
2. Make content optional as a part of selective disclosure.
3. Create 3 types of revision: content-only, signature-only, or witness-only.
4. Do it the way Git does it with GPG signature, where the signature envelopes the commit hash, but not a part of it.