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