commands: introduce a new command to edit commit metadata This patch introduces metaedit, a command to metadata of a set of revisions without updating the working copy. This is particularly relevant for repositories where changing the parent of the working copy is time-consuming. We could add more stack manipulation operations to metaedit in the future.

evolve: factor out check for creating unstable commits This check is pretty non-trivial, and we do it in two places already. We're going to do it in a third place soon.