blackbox: remove _bbvfs state

blackbox: remove _bbvfs state

`_bbvfs` is redundant because it could be calcualted from `_bbrepo`.

Differential Revision: https://phab.mercurial-scm.org/D651

--- a/hgext/blackbox.py	Wed Sep 06 21:08:59 2017 -0700
+++ b/hgext/blackbox.py	Wed Sep 06 21:12:27 2017 -0700
@@ -82,19 +82,23 @@
             else:
                 self._bbinlog = False
                 self._bbrepo = getattr(src, '_bbrepo', None)
-                self._bbvfs = getattr(src, '_bbvfs', None)
 
         def _partialinit(self):
-            if util.safehasattr(self, '_bbvfs'):
+            if self._bbvfs:
                 return
             self._bbinlog = False
             self._bbrepo = None
-            self._bbvfs = None
 
         def copy(self):
             self._partialinit()
             return self.__class__(self)
 
+        @property
+        def _bbvfs(self):
+            repo = getattr(self, '_bbrepo', None)
+            if repo:
+                return repo.vfs
+
         @util.propertycache
         def track(self):
             return self.configlist('blackbox', 'track', ['*'])
@@ -194,7 +198,6 @@
         def setrepo(self, repo):
             self._bbinlog = False
             self._bbrepo = repo
-            self._bbvfs = repo.vfs
 
     ui.__class__ = blackboxui
     uimod.ui = blackboxui