git-remote-testgit: fix error handling

git-remote-testgit: fix error handling

If fast-export did not complete successfully the error handling code
itself would error out.

This was broken in commit 23b093ee0 (Brandon Casey, Wed Jun 9 2010,
Remove python 2.5'isms). Revert that commit an introduce our own copy
of check_call in util.py instead.

Tested by changing 'if retcode' to 'if not retcode' temporarily.

Signed-off-by: Sverre Rabbelier <srabbelier@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>