shelve: Add Shelf.changed_files for resolving changed files in a plugin.

In a shelve plugin downstream, it's necessary to calculate the "files" changed by a shelf (for purposes of widening a narrow spec). It would be nice if a Shelf object could present an abstraction for this functionality.

In this draft, I've supplied an implementation that works downstream.

Questions about the approach: what's a good way to protect this functionality from erosion/removal since it's not reached by the public interface? Would a comment be sufficient? Or would a unit test be better? Does Mercurial offer unit tests?

Thanks for any advice.