git-send-email.perl: make initial In-Reply-To apply only to first email

When an initial --in-reply-to is supplied, make it apply only to the
first message; --[no-]chain-reply-to setting are honored by second and
subsequent messages; this is also how the git-format-patch option with
the same name behaves.

Moreover, when $initial_reply_to is asked to the user interactively it
is asked as the "Message-ID to be used as In-Reply-To for the _first_
email", this makes the user think that the second and subsequent
patches are not using it but are considered as replies to the first
message or chained according to the --[no-]chain-reply setting.

Look at the v2 series in the illustration to see what the new behavior
ensures:

       (before the patch)          |      (after the patch)
 [PATCH 0/2] Here is what I did... | [PATCH 0/2] Here is what I did...
   [PATCH 1/2] Clean up and tests  |   [PATCH 1/2] Clean up and tests
   [PATCH 2/2] Implementation      |   [PATCH 2/2] Implementation
   [PATCH v2 0/3] Here is a reroll |   [PATCH v2 0/3] Here is a reroll
   [PATCH v2 1/3] Clean up         |     [PATCH v2 1/3] Clean up
   [PATCH v2 2/3] New tests        |     [PATCH v2 2/3] New tests
   [PATCH v2 3/3] Implementation   |     [PATCH v2 3/3] Implementation

This is the typical behaviour we want when we send a series with cover
letter in reply to some discussion, the new patch series should appear
as a separate subtree in the discussion.

Also update the documentation on --in-reply-to to describe the new
behavior.

Signed-off-by: Antonio Ospite <ospite@studenti.unina.it>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -1319,7 +1319,8 @@ foreach my $t (@files) {
 
 	# set up for the next message
 	if ($thread && $message_was_sent &&
-		(chain_reply_to() || !defined $reply_to || length($reply_to) == 0)) {
+		(chain_reply_to() || !defined $reply_to || length($reply_to) == 0 ||
+		$message_num == 1)) {
 		$reply_to = $message_id;
 		if (length $references > 0) {
 			$references .= "\n $message_id";