swap: fix list element order on revert

[gaoflow]

Summary

revert() restores list elements in the wrong order when three or more elements are removed from a list, silently corrupting data and breaking the documented revert(diff(a, b), b) == a round-trip:

from dictdiffer import diff, revert
revert(diff({'x': ['p', 'q', 'r']}, {'x': []}), {'x': []})
# -> {'x': ['p', 'r', 'q']}   (expected {'x': ['p', 'q', 'r']})

It only manifests with ≥3 removed elements (with ≤2 the order happens to coincide), which is why the existing test_revert (2 elements) doesn't catch it.

Cause

diff emits list removals in descending index order so that patch's del dest[key] stays valid as the list shrinks. swap turns those remove entries into add entries, which revert/patch replay with dest.insert(key, value) — but inserting at descending indices into a growing list scrambles the order; insert-replay needs ascending order.

swap's add handler already reverses its changes (list(reversed(changes))); the remove handler omitted the symmetric reversal. That asymmetry is the bug.

Fix

Make swap's remove handler symmetric with add:

def remove(node, changes):
    return ADD, node, list(reversed(changes))

Harmless for dict removals (assignment order is irrelevant) and sets (single entry); only list inserts depend on order.

Verified: the minimal case and a 2000-case revert/patch round-trip fuzz now report 0 mismatches; dict and set reverts unchanged. Added a regression test (full removal and partial removal of ≥3 elements) that fails before the fix. Full unit suite: 88 passed, 2 skipped; the swap doctest passes. The changed line is pycodestyle-clean.

This pull request was prepared with the assistance of AI, under my direction and review.