metaedit: add support for folding commits while editing their metadata This also allows us to accept multiple commits without dealing with the thorny algorithmic and UI issues of editing multiple commits at once. Crucially, it is different from 'hg fold --exact' in that it also allows 'folding' a single commit and rewriting its metadata. This is really useful to have as a single logical operation, for example while preparing a series of multiple local changesets that will need to be pushed as a single changeset.

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.