The main difference is spreading out the control of the database to multiple actors. No single entity is allowed to make unilateral changes to the state of the database.
Moreover, the line between permisisonless and permissioned blockchains isn't necessarily about who's allowed to join but rather which identity they use. For RRR, the only reason we limit ourselves to permissioned is because we require long lived identities to prevent sybil attacks; otherwise it's as "open" as any public blockchain.
Git almost qualifies, but it has no automatic conflict resolution mechanism. Imagine how you would program a bank using git as a database. You would put each account balance in its own file and install a hook that checks that no money is created out of thin air. Commits are transactions. Everything kind of works, but how would you prevent double spends in such a system? Double spends are like divergent branches so presumably someone will need to step in and perform a merge. But this someone then becomes a point of centralization, thereby disqualifying git as a blockchain ;)
Git solves a different problem. Look up state machine replication, and then think about the case where some portion of the participants (max 1/3) are malicious. In git how do you deal where there is no central 'trusted repo' to push to, and a malicious participant sends a
commit to some subset of participants but not to others (i.e. equivocates)?
Moreover, the line between permisisonless and permissioned blockchains isn't necessarily about who's allowed to join but rather which identity they use. For RRR, the only reason we limit ourselves to permissioned is because we require long lived identities to prevent sybil attacks; otherwise it's as "open" as any public blockchain.