scmutil.readonlyvfs: implement join

scmutil.readonlyvfs: implement join

Required in an upcoming patch.

--- a/mercurial/scmutil.py	Wed Sep 02 22:05:23 2015 -0700
+++ b/mercurial/scmutil.py	Wed Sep 02 22:05:58 2015 -0700
@@ -586,6 +586,8 @@
             raise util.Abort('this vfs is read only')
         return self.vfs(path, mode, *args, **kw)
 
+    def join(self, path, *insidef):
+        return self.vfs.join(path, *insidef)
 
 def walkrepos(path, followsym=False, seen_dirs=None, recurse=False):
     '''yield every hg repository under path, always recursively.