add--interactive: handle EOF in prompt_yesno

The prompt_yesno function loops indefinitely waiting for a "y" or "n" response. But it doesn't handle EOF, meaning that we can end up in an infinite loop of reading EOF from stdin. One way to simulate that is with: echo e | GIT_EDITOR='echo corrupt >' git add -p Let's break out of the loop and propagate the undef to the caller. Without modifying the callers that effectively turns it into a "no" response. This is reasonable for both of the current callers, and it leaves room for any future caller to check for undef explicitly. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>