interactive: explicitly `fflush` stdout before expecting input

interactive: explicitly `fflush` stdout before expecting input

At least one interactive command writes a prompt to `stdout` and then
reads user input on `stdin`: `git clean --interactive`. If the prompt is
left in the buffer, the user will not realize the program is waiting for
their input.

So let's just flush `stdout` before reading the user's input.

Signed-off-by: 마누엘 <redacted>
Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>