Redone commits keep their checkpoint trailers

An agent's commits redone as logical ones with git reset and new commits lost every trailer, so the session vanished from the trail. The dropped commits are those between the merge base of HEAD and ORIG_HEAD and ORIG_HEAD itself; a dropped commit's trailers are carried into the new message when a file it changed is staged with exactly the content it had at ORIG_HEAD. Rewritten content inherits nothing, a merge or pull replaces nothing, and the reset must still be the latest ref operation in HEAD's reflog. Renames, deletions and file/directory swaps are matched by the tree changes they make.