combine_notes_cat_sort_uniq(): sort and dedup lines all at once

combine_notes_cat_sort_uniq(): sort and dedup lines all at once

Instead of reading lines one by one and insertion-sorting them into a
string_list, read all of the lines, sort them, then remove duplicates.
Aside from being less code, this reduces the complexity from O(N^2) to
O(N lg N) in the total number of lines.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Acked-by: Johan Herland <johan@herland.net>
Signed-off-by: Jeff King <peff@peff.net>