send-email: fix In-Reply-To regression

send-email: fix In-Reply-To regression

Fix a regression introduced by

1ca3d6e (send-email: squelch warning due to comparing undefined $_ to "")

where if the user was prompted for an initial In-Reply-To and didn't
provide one, messages would be sent out with an invalid In-Reply-To of
"<>"

Also add test cases for the regression and the fix. A small modification
was needed to allow send-email to take its replies from stdin if the
environment variable GIT_SEND_EMAIL_NOTTY is set.

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