am: simplify prompt response handling

am: simplify prompt response handling

We'll never see a NULL returned from git_prompt(); if it can't produce
any input for us (e.g., because the terminal got EOF) then it will just
die().

So there's no need for us to handle NULL here. And even if there was, it
doesn't make sense to continue; on a true terminal hangup we'd just loop
infinitely trying to get input that will never come.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>