Re: [PATCH 9/9] update-ref: implement interactive transaction handling

> The git-update-ref(1) command can only handle queueing transactions
> right now via its "--stdin" parameter, but there is no way for users to
> handle the transaction itself in a more explicit way. E.g. in a
> replicated scenario, one may imagine a coordinator that spawns
> git-update-ref(1) for multiple repositories and only if all agree that
> an update is possible will the coordinator send a commit. Such a
> transactional session could look like
>
>     > start
>     < start: ok
>     > update refs/heads/master $OLD $NEW
>     > prepare
>     < prepare: ok
>     # All nodes have returned "ok"
>     > commit
>     < commit: ok
>
> or
>
>     > start
>     < start: ok
>     > create refs/heads/master $OLD $NEW
>     > prepare
>     < fatal: cannot lock ref 'refs/heads/master': reference already exists
>     # On all other nodes:
>     > abort
>     < abort: ok
>
> In order to allow for such transactional sessions, this commit
> introduces four new commands for git-update-ref(1), which matches those
> we have internally already with the exception of "start":
>
>     - start: start a new transaction
>
>     - prepare: prepare the transaction, that is try to lock all
>                references and verify their current value matches the
>                expected one
>
>     - commit: explicitly commit a session, that is update references to
>               match their new expected state
>
>     - abort: abort a session and roll back all changes