Merge branch 'en/ort-harden-against-corrupt-trees' into jch

Merge branch 'en/ort-harden-against-corrupt-trees' into jch

"ort" merge backend handles merging corrupt trees better by
aborting when it should.

* en/ort-harden-against-corrupt-trees:
  cache-tree: fix verify_cache() to catch non-adjacent D/F conflicts
  merge-ort: abort merge when trees have duplicate entries
  merge-ort: free diff pairs queue in clear_or_reinit_internal_opts()
  merge-ort: drop unnecessary show_all_errors from collect_merge_info()
  merge-ort: propagate callback errors from traverse_trees_wrapper()