pycompat: introduce an alias for urllib.unquote

We have to use unquote_to_bytes on Python 3, so we need an abstraction
for this.

mercurial/pycompat.py

@@ -22,6 +22,7 b' if not ispy3:'
     import Queue as _queue
     import SocketServer as socketserver
     import urlparse
+    urlunquote = urlparse.unquote
     import xmlrpclib
 else:
     import http.client as httplib
@@ -30,6 +31,7 b' else:'
     import queue as _queue
     import socketserver
     import urllib.parse as urlparse
+    urlunquote = urlparse.unquote_to_bytes
     import xmlrpc.client as xmlrpclib
 
 if ispy3: