send-email: fix nasty bug in ask() function

send-email: fix nasty bug in ask() function

Commit 6e18251 (send-email: refactor and ensure prompting doesn't loop
forever) introduced an ask function, which unfortunately had a nasty
bug. This caused it not to accept anything but the default reply to the
"Who should the emails appear to be from?" prompt, and nothing but
ctrl-d to the "Who should the emails be sent to?" and "Message-ID to be
used as In-Reply-To for the first email?" prompts.

This commit corrects the issues and adds a test to confirm the fix.

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>