[PATCH STABLE] localrepo: clear filecache correctly on destroyed()

[Hao Lian]

Good idea!

> ...
>  r.invalidate()          # try to drop cache
> ...
> This doesn't actually trigger a bug for me though.

Between 9f94358 and my patch, invalidate() no longer clears the
filecache. `r._filecache.clear()` would work though, or something
equivalent (like calling `r.rollback`). I'll add that as a test in the
next patch.