git send-email: ask less questions when --compose is used.

git send-email: ask less questions when --compose is used.

When --compose is used, we can grab the From/Subject/In-Reply-To from the
edited summary, let it be so and don't ask the user silly questions.

The summary templates gets quite revamped, and includes the list of
patches subjects that are going to be sent with this batch.

When having a body full of empty lines, the summary isn't sent. Document
that in the git-send-email manpage fully.

Note: It doesn't deal with To/Cc/Bcc yet.

Signed-off-by: Pierre Habouzit <madcoder@debian.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>