t4014: let sed open its own files

t4014: let sed open its own files

In some cases, we were using a redirection operator to feed input into
sed. However, since sed is capable of opening its own files, make sed
open its own files instead of redirecting input into it.

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