url: avoid re-issuing incorrect password (issue3210)

Some draconian IT setups lock accounts after a small number of incorrect
password attempts. Mercurial's implementation of the urllib2 authentication was
causing 5 retry attempts with the same credentials, without prompting the user.
The code was attempting to check whether the authorization token had changed,
but unfortunately was reading the misleading 'headers' member of the request
instead of using the 'get_header' accessor.

Modelled on fix for Python issue 8797:
https://bugs.python.org/issue8797
https://hg.python.org/cpython/rev/30e8a8f22a2a

mercurial/url.py

@@ -451,7 +451,7 @@ class httpbasicauthhandler(urlreq.httpbasicauthhandler):
         if pw is not None:
             raw = "%s:%s" % (user, pw)
             auth = 'Basic %s' % base64.b64encode(raw).strip()
-            if req.headers.get(self.auth_header, None) == auth:
+            if req.get_header(self.auth_header, None) == auth:
                 return None
             self.auth = auth
             req.add_unredirected_header(self.auth_header, auth)