Merge branch 'jk/fetch-pack-remove-dups-optim'

The way "fetch-pack" that is given multiple references to fetch tried to remove duplicates was very inefficient. By Jeff King * jk/fetch-pack-remove-dups-optim: fetch-pack: sort incoming heads list earlier fetch-pack: avoid quadratic loop in filter_refs fetch-pack: sort the list of incoming refs add sorting infrastructure for list refs fetch-pack: avoid quadratic behavior in remove_duplicates fetch-pack: sort incoming heads