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.