merge-recursive.c: use string_list_sort instead of qsort

merge-recursive.c: use string_list_sort instead of qsort

Merge-recursive sorts a string list using a raw qsort(), where it
feeds the "items" from one struct but the "nr" and size fields from
another struct. This isn't a bug because one list is a copy of the
other, but it's unnecessarily confusing (and also caused our recent
QSORT() cleanups via coccinelle to miss this call site).

Let's use string_list_sort() instead, which is more concise and harder
to get wrong. Note that we need to adjust our comparison function,
which gets fed only the strings now, not the string_list_items. That's
OK because we don't use the "util" field as part of our sort.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>