url: Remove the proxy env variables only when needed (issue2451)

This is an attempt to fix issue 2451 and its duplicates (2599 and 2949, AFAIK).
Its main idea is that it is only necessary to clean the proxy environment
variables *when* http_proxy is set in the config file (since it takes
precedence over the environment variables). Otherwise, hg shouldn't bother with
them, since they will most likely be used to reach the server.

mercurial/url.py

@@ -93,7 +93,9 b' class proxyhandler(urllib2.ProxyHandler)'
             proxies = {}
 
         # urllib2 takes proxy values from the environment and those
-        # will take precedence if found, so drop them
+        # will take precedence if found. So, if there's a config entry
+        # defining a proxy, drop the environment ones
+        if ui.config("http_proxy", "host"):
         for env in ["HTTP_PROXY", "http_proxy", "no_proxy"]:
             try:
                 if env in os.environ: