Merge branch 'mh/notes-string-list'

Merge branch 'mh/notes-string-list'

Improve the asymptotic performance of the cat_sort_uniq notes merge
strategy.

* mh/notes-string-list:
  string_list_add_refs_from_colon_sep(): use string_list_split()
  notes: fix handling of colon-separated values
  combine_notes_cat_sort_uniq(): sort and dedup lines all at once
  Initialize sort_uniq_list using named constant
  string_list: add a function string_list_remove_empty_items()