t1404: do not rely on the exact phrasing of strerror()

Not even in C locale, it is wrong to expect that the exact phrasing
"File exists" is used to show EEXIST.

Reported-by: Randall S. Becker <rsbecker@nexbridge.com>
Helped-by: Duy Nguyen <pclouds@gmail.com>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1404-update-ref-errors.sh

@@ -614,7 +614,7 @@ test_expect_success 'delete fails cleanly if packed-refs file is locked' '
 	test_when_finished "rm -f .git/packed-refs.lock" &&
 	test_must_fail git update-ref -d $prefix/foo >out 2>err &&
 	git for-each-ref $prefix >actual &&
-	test_i18ngrep "Unable to create $Q.*packed-refs.lock$Q: File exists" err &&
+	test_i18ngrep "Unable to create $Q.*packed-refs.lock$Q: " err &&
 	test_cmp unchanged actual
 '