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>