monotone compile fix

#
# old_revision [2dbb3ad115d10494b2841b26296275f92512c8f4]
#
# patch "cmd_key_cert.cc"
#  from [488ff4175a9c2b0cf85ed15fdf16071e60c60f44]
#    to [75c7f1f9de2b1c850deb13ea6a40a8a09c8eb008]
#
============================================================
--- cmd_key_cert.cc     488ff4175a9c2b0cf85ed15fdf16071e60c60f44
+++ cmd_key_cert.cc     75c7f1f9de2b1c850deb13ea6a40a8a09c8eb008
@@ -150,7 +150,8 @@ CMD(ssh_agent_export, N_("key and cert")
     cout << decoded_key;
   else
     {
-      ofstream fout(system_path(idx(args, 0)).as_external().c_str(), ofstream::out);
+      string path = system_path(idx(args, 0)).as_external();
+      ofstream fout(path.c_str(), ofstream::out);
       fout << decoded_key;
     }
 }