Tell a redo from a coincidence

The redo rule matched too loosely and missed real redos:

• Commits still on a branch or remote were "dropped": merging main and then reset --soft main inherited main's history, and undoing a rebase inherited the teammate commit it had brought in. Dropped now means only the reset's old tip reaches it.
• One byte-identical file (a lockfile, an empty init.py, the same deletion) among rewritten ones inherited a discarded attempt. Every staged file a dropped commit changed must now match.
• Ordinary commits never closed the window. Commits since the reset now keep it open only while each redid dropped work.
• Unstaging or a stash after reset --soft overwrote ORIG_HEAD and lost the redo, and an amend fold never looked. The reset is read from HEAD's reflog, skipping resets that did not move HEAD, and amends inherit too.