add--interactive: handle EOF in prompt_yesno

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>