imap-send: use git-credential

git-imap-send was directly prompting for a password rather than using
git-credential. git-send-email, on the other hand, supports git-credential.

This is a necessary improvement for users that use two factor authentication, as
they should not be expected to remember all of their app specific passwords.

Signed-off-by: Dan Albert <danalbert@google.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>