Merge branch 'jh/notes-merge' into next

Merge branch 'jh/notes-merge' into next

* jh/notes-merge: (23 commits)
  Provide 'git merge --abort' as a synonym to 'git reset --merge'
  cmd_merge(): Parse options before checking MERGE_HEAD
  Provide 'git notes get-ref' to easily retrieve current notes ref
  git notes merge: Add testcases for merging notes trees at different fanouts
  git notes merge: Add another auto-resolving strategy: "cat_sort_uniq"
  git notes merge: --commit should fail if underlying notes ref has moved
  git notes merge: List conflicting notes in notes merge commit message
  git notes merge: Manual conflict resolution, part 2/2
  git notes merge: Manual conflict resolution, part 1/2
  Documentation: Preliminary docs on 'git notes merge'
  git notes merge: Add automatic conflict resolvers (ours, theirs, union)
  git notes merge: Handle real, non-conflicting notes merges
  builtin/notes.c: Refactor creation of notes commits.
  git notes merge: Initial implementation handling trivial merges only
  builtin/notes.c: Split notes ref DWIMmery into a separate function
  notes.c: Use two newlines (instead of one) when concatenating notes
  (trivial) t3303: Indent with tabs instead of spaces for consistency
  notes.h/c: Propagate combine_notes_fn return value to add_note() and beyond
  notes.h/c: Allow combine_notes functions to remove notes
  notes.c: Reorder functions in preparation for next commit
  ...

Conflicts:
	builtin.h