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.

evolve: factor out sanity checks for folds We're going to use the same checks in another context in an upcoming patch.

evolve: include node, rev, desc, and phase in list

evolve: use hex in list output

py3: use items() instead of iteritems()