string-list: add string_list_sort_u() that mimics "sort -u"

string-list: add string_list_sort_u() that mimics "sort -u"

Many callsites of string_list_remove_duplicates() call it
immdediately after calling string_list_sort(), understandably
as the former requires string-list to be sorted, it is clear
that these places are sorting only to remove duplicates and
for no other reason.

Introduce a helper function string_list_sort_u that combines
these two calls that often appear together, to simplify
these callsites. Replace the current calls of those methods with
string_list_sort_u().

Signed-off-by: Amisha Chhajed <amishhhaaaa@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>