_git_utils: Skip track_want when tracking is not set

When exact_want fails with "not our ref", the fetch falls back to track_want which passes track to get_matching_refs and then to pattern_to_regex. If tracking is not configured, this raises a NoneType AttributeError in pattern_to_regex. So skip the track_want entirely when track is not set and proceed directly to the full fetch.

Fixes: #91