histedit: store backup file before histedit It's possible for the user to delete some of the commits they started with during a histedit, and aborting the histedit doesn't bring them back. Let's store a backup bundle so we can always recover the stack of commits from before they began.

histedit: replace pickle with custom serialization Pickle is undesirable, so let's serialize it ourselves. We keep the ability to parse existing pickle blobs for now.

devel-warn: add a prefix to all messages ("devel-warn: ") We want to make the origin and importance of the message clear to developers.

push: acquire local 'wlock' if "pushback" is expected (BC) (issue4596) If the client allows "pushback", the bundle2 served back by the server may contains parts that will write to the repository. Such parts may require the 'wlock' (eg: bookmark) so we acquire it in advance to make sure it got acquired before the 'lock'.