format-patch: thread as reply to cover letter even with in-reply-to

Currently, format-patch --thread --cover-letter --in-reply-to $parent
makes all mails, including the cover letter, a reply to $parent.
However, we would want the reader to consider the cover letter above
all the patches.

This changes the semantics so that only the cover letter is a reply to
$parent, while all the patches are formatted as replies to the cover
letter.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-log.c

@@ -1036,12 +1036,22 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 			/* Have we already had a message ID? */
 			if (rev.message_id) {
 				/*
-				 * If we've got the ID to be a reply
-				 * to, discard the current ID;
-				 * otherwise, make everything a reply
-				 * to that.
+				 * Without --cover-letter and
+				 * --in-reply-to, make every mail a
+				 * reply to the one before.
+				 *
+				 * With --in-reply-to but no
+				 * --cover-letter, make every mail a
+				 * reply to the <reply-to>.
+				 *
+				 * With --cover-letter, make every
+				 * mail but the cover letter a reply
+				 * to the cover letter.  The cover
+				 * letter is a reply to the
+				 * --in-reply-to, if specified.
 				 */
-				if (rev.ref_message_ids->nr > 0)
+				if (rev.ref_message_ids->nr > 0
+				    && (!cover_letter || rev.nr > 1))
 					free(rev.message_id);
 				else
 					string_list_append(rev.message_id,

t/t4014-format-patch.sh

@@ -237,16 +237,19 @@ In-Reply-To: <1>
 References: <1>
 ---
 Message-Id: <2>
-In-Reply-To: <1>
+In-Reply-To: <0>
 References: <1>
+	<0>
 ---
 Message-Id: <3>
-In-Reply-To: <1>
+In-Reply-To: <0>
 References: <1>
+	<0>
 ---
 Message-Id: <4>
-In-Reply-To: <1>
+In-Reply-To: <0>
 References: <1>
+	<0>
 EOF
 
 test_expect_success 'thread cover-letter in-reply-to' '