Merge branch 'en/merge-restore-to-pristine'

When "git merge" finds that it cannot perform a merge, it should
restore the working tree to the state before the command was
initiated, but in some corner cases it didn't.

* en/merge-restore-to-pristine:
  merge: do not exit restore_state() prematurely
  merge: ensure we can actually restore pre-merge state
  merge: make restore_state() restore staged state too
  merge: fix save_state() to work when there are stat-dirty files
  merge: do not abort early if one strategy fails to handle the merge
  merge: abort if index does not match HEAD for trivial merges
  merge-resolve: abort if index does not match HEAD
  merge-ort-wrappers: make printed message match the one from recursive