Comment 2 for bug 401249

Okay, I've written a test on branch lp:~naesten/bzr-fastimport/401249-tag-from-ref.

As for a fix, I'm looking at the code, and it looks to me as though there ought to be a method on cache_manager.CacheManager to look up a committish -- as it stands, the logic seems to be tied up in the head tracking code, which is *probably* not something we want to call from GenericProcessor._set_tag(), though I'm not positive about that.