fetch: Speed up fetch by rewriting find_non_local_tags

fetch: Speed up fetch by rewriting find_non_local_tags

When trying to get a list of remote tags to see if we need to fetch
any we were doing a linear search for the matching tag ref for the
tag^{} commit entries.  This proves to be incredibly slow for large
numbers of tags.  Rewrite the function so that we build up a
string_list of refs to fetch and then process that instead.

As an extreme example, for a repository with 50000 tags (and just a
single commit on a single branch), a fetch that does nothing goes from
~1m50s to ~4.1s.

Signed-off-by: Julian Phillips <julian@quantumfyre.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>