prune_remote(): sort delete_refs_list references en masse

prune_remote(): sort delete_refs_list references en masse

Inserting items into a list in sorted order is O(N^2) whereas
appending them unsorted and then sorting the list all at once is
O(N lg N).

string_list_insert() also removes duplicates, and this change loses
that functionality. But the strings in this list, which ultimately
come from a for_each_ref() iteration, cannot contain duplicates.

Signed-off-by: Michael Haggerty <redacted>
Reviewed-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>