submodule.c: sort changed_submodule_names before searching it

submodule.c: sort changed_submodule_names before searching it

We can string_list_insert() to maintain sorted-ness of the
list as we find new items, or we can string_list_append() to
build an unsorted list and sort it at the end just once.

As we do not rely on the sortedness while building the
list, we pick the "append and sort at the end" as it
has better worst case execution times.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>