oid-array: make sort function public

oid-array: make sort function public

We sort the oid-array as a side effect of calling the lookup or
unique-iteration functions. But callers may want to sort it themselves
(especially as we add new iteration options in future patches).

We'll also move the check of the "sorted" flag into the sort function,
so callers don't have to remember to check it.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>