url: fix bug in passwordmgr related to auth configuration

Usernames given as part of the URL would be ignored.
This bug was introduced in 89c80c3dc5842d609e9ba13f08fd2cf7f127c25a

mercurial/url.py

@@ -108,7 +108,8 b' class passwordmgr(urllib2.HTTPPasswordMg'
             self._writedebug(user, passwd)
             return (user, passwd)
 
-        user, passwd = self._readauthtoken(authuri)
+        if not user:
+            user, passwd = self._readauthtoken(authuri)
         if not user or not passwd:
             if not self.ui.interactive():
                 raise util.Abort(_('http authorization required'))