Allow builtin-fetch's find_non_local_tags to append onto a list

Allow builtin-fetch's find_non_local_tags to append onto a list

By allowing the function to append onto the end of an existing list
we can do more interesting things, like join the list of tags we
want to fetch into the first fetch, rather than the second.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>