Merge branch 'hn/git-checkout-m-with-stash' into next

Merge branch 'hn/git-checkout-m-with-stash' into next

"git checkout -m another-branch" was invented to deal with local
changes to paths that are different between the current and the new
branch, but it gave only one chance to resolve conflicts.  The command
was taught to create a stash to save the local changes.

* hn/git-checkout-m-with-stash:
  checkout: -m (--merge) uses autostash when switching branches
  sequencer: teach autostash apply to take optional conflict marker labels
  sequencer: allow create_autostash to run silently
  stash: add --label-ours, --label-theirs, --label-base for apply