url: always access req._tunnel_host

The getattr() was there to handle Python versions before 2.6, which
lacked this attribute.

We /might/ be able to further delete some code here. However, the
behavior here is extremely hard to follow because large parts of
this code duplicate code from the Python standard library and it
is difficult to understand what is actually needed.

Differential Revision: https://phab.mercurial-scm.org/D5950

mercurial/url.py

@@ -179,10 +179,10 b' class httpconnection(keepalive.HTTPConne'
             return proxyres
         return keepalive.HTTPConnection.getresponse(self)
 
-# general transaction handler to support different ways to handle
-# HTTPS proxying before and after Python 2.6.3.
+# Large parts of this function have their origin from before Python 2.6
+# and could potentially be removed.
 def _generic_start_transaction(handler, h, req):
-    tunnel_host = getattr(req, '_tunnel_host', None)
+    tunnel_host = req._tunnel_host
     if tunnel_host:
         if tunnel_host[:7] not in ['http://', 'https:/']:
             tunnel_host = 'https://' + tunnel_host