url: use rsplit to split [auth] keys

None of the auth section subkeys include a period, so it makes zero
sense to not split from the end.  By using rsplit() users can use
the hostname as group keys.

mercurial/url.py

@@ -155,7 +155,7 class passwordmgr(urllib2.HTTPPasswordMg
             if '.' not in key:
                 self.ui.warn(_("ignoring invalid [auth] key '%s'\n") % key)
                 continue
-            group, setting = key.split('.', 1)
+            group, setting = key.rsplit('.', 1)
             gdict = config.setdefault(group, dict())
             if setting in ('username', 'cert', 'key'):
                 val = util.expandpath(val)