A  `unfilteredpropertycache` is a kind of `propertycache` used on `localrepo` to
unsure it will always be run against unfiltered repo and stored only once.

As the cached value is never stored in the repoview instance, the descriptor
will always be called. Before this patch such calls always result in a call to
the `__get__` method of the `propertycache` on the unfiltered repo. That was
recomputing a new value on every access through a repoview.

We can't prevent the repoview's `unfilteredpropertycache` to get called on every
access. In that case the new code makes a standard attribute access to the
property. If a value is cached it will be used.

The `propertycache` test file have been augmented with test about this issue.