pathutil: restate dirname and join as forwards to posixpath

I've done this as its own step so that it's easy to see that the
posixpath implementations pass the doctests in this package. In a
future patch I'll just make these pure forwards of the methods so that
things using pathutil can be oblivious to the posix nature of these
functions.

mercurial/pathutil.py

@@ -1,4 +1,4
-import os, errno, stat
+import os, errno, stat, posixpath
 
 import encoding
 import util
@@ -188,7 +188,7 def normasprefix(path):
     else:
         return path
 
-def join(path, *paths):
+def join(*args):
     '''Join two or more pathname components, inserting '/' as needed.
 
     Based on the posix os.path.join() implementation.
@@ -214,17 +214,7 def join(path, *paths):
     >>> join ('foo', '', '', 'bar')
     'foo/bar'
     '''
-    sep = '/'
-    if not paths:
-        path[:0] + sep #23780: Ensure compatible data type even if p is null.
-    for piece in paths:
-        if piece.startswith(sep):
-            path = piece
-        elif not path or path.endswith(sep):
-            path += piece
-        else:
-            path += sep + piece
-    return path
+    return posixpath.join(*args)
 
 def dirname(path):
     '''returns the directory portion of the given path
@@ -246,9 +236,4 def dirname(path):
     >>> dirname('/foo//bar')
     '/foo'
     '''
-    sep = '/'
-    i = path.rfind(sep) + 1
-    dirname = path[:i]
-    if dirname and dirname != sep * len(dirname):
-        dirname = dirname.rstrip(sep)
-    return dirname
+    return posixpath.dirname(path)