remote.c: make singular free_ref() public

remote.c: make singular free_ref() public

We provide a free_refs() function to free a list, but there's no easy
way for a caller to free a single ref. Let's make our singular
free_ref() function public. Since its name is so similar to the
list-freeing free_refs(), and because both of those functions have the
same signature, it might be easy to accidentally use the wrong one.
Let's call the singular version the more verbose "free_one_ref()" to
distinguish it.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>