vfs: define "join()" in each classes derived from "abstractvfs"

This patch defines "join()" in each classes derived from "abstractvfs"
except "vfs", which already defines it.

This allows all vfs instances to be used for indirect file API
invocation.

mercurial/scmutil.py

@@ -347,6 +347,12 b' class filtervfs(abstractvfs):'
     def __call__(self, path, *args, **kwargs):
         return self._orig(self._filter(path), *args, **kwargs)
 
+    def join(self, path):
+        if path:
+            return self._orig.join(self._filter(path))
+        else:
+            return self._orig.join(path)
+
 filteropener = filtervfs
 
 def canonpath(root, cwd, myname, auditor=None):

mercurial/statichttprepo.py

@@ -74,6 +74,12 b' def build_opener(ui, authinfo):'
             f = "/".join((self.base, urllib.quote(path)))
             return httprangereader(f, urlopener)
 
+        def join(self, path):
+            if path:
+                return os.path.join(self.base, path)
+            else:
+                return self.base
+
     return statichttpvfs
 
 class statichttppeer(localrepo.localpeer):

mercurial/store.py

@@ -441,6 +441,12 b' class _fncachevfs(scmutil.abstractvfs):'
             self.fncache.add(path)
         return self.vfs(self.encode(path), mode, *args, **kw)
 
+    def join(self, path):
+        if path:
+            return self.vfs.join(self.encode(path))
+        else:
+            return self.vfs.join(path)
+
 class fncachestore(basicstore):
     def __init__(self, path, vfstype, dotencode):
         if dotencode: