scmutil: allow access to filecache descriptor on class

To make it easier to patch the wrapped function, make it possible to access the
filecache descriptor directly on the class (rather than have to use
ClassObject.__dict__['attributename']). Returning `self` when the first
argument to `__get__` is `None` makes the descriptor behave the same way
`property` objects do.

mercurial/localrepo.py

@@ -66,6 +66,8 b' class repofilecache(scmutil.filecache):'
     """
 
     def __get__(self, repo, type=None):
+        if repo is None:
+            return self
         return super(repofilecache, self).__get__(repo.unfiltered(), type)
     def __set__(self, repo, value):
         return super(repofilecache, self).__set__(repo.unfiltered(), value)

mercurial/scmutil.py

@@ -1205,6 +1205,9 b' class filecache(object):'
         return self
 
     def __get__(self, obj, type=None):
+        # if accessed on the class, return the descriptor itself.
+        if obj is None:
+            return self
         # do we need to check if the file changed?
         if self.name in obj.__dict__:
             assert self.name in obj._filecache, self.name