blackbox: do not assume self._bb{vfs,repo,fp} are set in blackboxui.__init__

It's possible for the blackboxui code to do a "del self._bbvfs", then ui.copy()
or similar attempt will fail. It will also fail when constructing a blackboxui
from a non-blackbox ui.
This patch fixes the issue by not assuming any _bb* attr is set.

hgext/blackbox.py

@@ -80,10 +80,10 b' def wrapui(ui):'
             if src is None:
                 self._partialinit()
             else:
-                self._bbfp = src._bbfp
+                self._bbfp = getattr(src, '_bbfp', None)
                 self._bbinlog = False
-                self._bbrepo = src._bbrepo
-                self._bbvfs = src._bbvfs
+                self._bbrepo = getattr(src, '_bbrepo', None)
+                self._bbvfs = getattr(src, '_bbvfs', None)
 
         def _partialinit(self):
             if util.safehasattr(self, '_bbvfs'):