Add missing return False to preconnect

Noticed by users on repo-discuss, we were missing a return False
here to signal that SSH control master was not used to setup the
network connection.

Signed-off-by: Shawn O. Pearce <sop@google.com>

git_config.py

@@ -414,6 +414,7 @@ def _preconnect(url):
     host = m.group(1)
     return _open_ssh(host, 22)
 
+  return False
 
 class Remote(object):
   """Configuration options related to a remote.