t1409: let sed open its own input file

t1409: let sed open its own input file

In one case, we were using a redirection operator to feed input into
sed. However, since sed is capable of opening its own input file, make
sed do that instead of redirecting input into it.

Signed-off-by: Denton Liu <redacted>
Signed-off-by: Junio C Hamano <redacted>