shelve: add a shelve extension to save/restore working changes This extension saves shelved changes using a temporary draft commit, and bundles the temporary commit and its draft ancestors, then strips them. This strategy makes it possible to use Mercurial's bundle and merge machinery to resolve conflicts if necessary when unshelving, even when the destination commit or its ancestors have been amended, squashed, or evolved. (Once a change has been unshelved, its associated unbundled commits are either rolled back or stripped.) Storing the shelved change as a bundle also avoids the difficulty that hidden commits would cause, of making it impossible to amend the parent if it is a draft commits (a common scenario). Although this extension shares its name and some functionality with the third party hgshelve extension, it has little else in common. Notably, the hgshelve extension shelves changes as unified diffs, which makes conflict resolution a matter of finding .rej files and conflict markers, and cleaning up the mess by hand. We do not yet allow hunk-level choosing of changes to record. Compared to the hgshelve extension, this is a small regression in usability, but we hope to integrate that at a later point, once the record machinery becomes more reusable and robust.