[PATCH v2] git-send-email.perl: fix In-Reply-To for second and subsequent patches

[Antonio Ospite <ospite@studenti.unina.it>]

Make second and subsequent patches appear as replies to the first patch,
even when an initial In-Reply-To is supplied; this is the typical
behaviour we want when we send a series with cover letter in reply to
some discussion, and this is also what the man page says about
the --in-reply-to option.

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.

In order to achieve the old behaviour of a flat structure in reply to
something the user can always use "--no-thread --in-reply-to <...>".

Signed-off-by: Antonio Ospite <ospite@studenti.unina.it>
---

Changes since v1:
 - add more details about the interactive case in the commit message
 - split long line as requested by Jonathan Nieder
 - add a test case in t/t9001-send-email.sh, please check that, I am not
   comparing the strings inside '<>' is it necessary to be so strict?
   Note to self: remember to run 'make' before running t9001-send-email.sh :)

Thanks,
   Antonio Ospite
   http://ao2.it

 git-send-email.perl   |    3 ++-
 t/t9001-send-email.sh |   14 ++++++++++++++
 2 files changed, 16 insertions(+), 1 deletions(-)

diff --git a/git-send-email.perl b/git-send-email.perl
index 8cc4161..bc4e318 100755
--- a/git-send-email.perl
+++ b/git-send-email.perl
@@ -1313,7 +1313,8 @@ 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";
diff --git a/t/t9001-send-email.sh b/t/t9001-send-email.sh
index a298eb0..410b85f 100755
--- a/t/t9001-send-email.sh
+++ b/t/t9001-send-email.sh
@@ -295,6 +295,20 @@ test_expect_success $PREREQ 'Valid In-Reply-To when prompting' '
 	! grep "^In-Reply-To: < *>" msgtxt1
 '
 
+test_expect_success $PREREQ 'In-Reply-To in second patch with --thread' '
+	clean_fake_sendmail &&
+	git send-email \
+		--from="Example <nobody@example.com>" \
+		--to=nobody@example.com \
+		--thread \
+		--in-reply-to="<unique-message-id@example.com>" \
+		--smtp-server="$(pwd)/fake.sendmail" \
+		$patches $patches \
+		2>errors
+        # The second patch should be seen as reply to the first one
+        test $(sed -n -e "s/^In-Reply-To:\(.*\)/\1/p" msgtxt2) = $(sed -n -e "s/^Message-Id:\(.*\)/\1/p" msgtxt1)
+'
+
 test_expect_success $PREREQ 'setup fake editor' '
 	(echo "#!$SHELL_PATH" &&
 	 echo "echo fake edit >>\"\$1\""
-- 
1.7.2.3