git-format-patch: Make the second and subsequent mails replies to the first

Add message_id and ref_message_id fields to struct rev_info, used in show_log
with CMIT_FMT_EMAIL to set Message-Id and In-Reply-To/References respectively.
Use these in git-format-patch to make the second and subsequent patch mails
replies to the first patch mail.
Signed-off-by: NJosh Triplett <josh@freedesktop.org>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

builtin-log.c

@@ -226,6 +226,17 @@ static void get_patch_ids(struct rev_info *rev, struct diff_options *options)
 	o2->flags = flags2;
 }
 
+static void gen_message_id(char *dest, unsigned int length, char *base)
+{
+	const char *committer = git_committer_info(1);
+	const char *email_start = strrchr(committer, '<');
+	const char *email_end = strrchr(committer, '>');
+	if(!email_start || !email_end || email_start > email_end - 1)
+		die("Could not extract email from committer identity.");
+	snprintf(dest, length, "%s.%u.git.%.*s", base, time(NULL),
+		 email_end - email_start - 1, email_start + 1);
+}
+
 int cmd_format_patch(int argc, const char **argv, char **envp)
 {
 	struct commit *commit;
@@ -239,6 +250,8 @@ int cmd_format_patch(int argc, const char **argv, char **envp)
 	int ignore_if_in_upstream = 0;
 	struct diff_options patch_id_opts;
 	char *add_signoff = NULL;
+	char message_id[1024];
+	char ref_message_id[1024];
 
 	git_config(git_format_config);
 	init_revisions(&rev);
@@ -365,6 +378,16 @@ int cmd_format_patch(int argc, const char **argv, char **envp)
 		int shown;
 		commit = list[nr];
 		rev.nr = total - nr + (start_number - 1);
+		/* Make the second and subsequent mails replies to the first */
+		if (nr == (total - 2)) {
+			strncpy(ref_message_id, message_id,
+				sizeof(ref_message_id));
+			ref_message_id[sizeof(ref_message_id)-1] = '\0';
+			rev.ref_message_id = ref_message_id;
+		}
+		gen_message_id(message_id, sizeof(message_id),
+			       sha1_to_hex(commit->object.sha1));
+		rev.message_id = message_id;
 		if (!use_stdout)
 			reopen_stdout(commit, rev.nr, keep_subject);
 		shown = log_tree_commit(&rev, commit);

log-tree.c

@@ -97,6 +97,11 @@ void show_log(struct rev_info *opt, const char *sep)
 			subject = "Subject: ";
 
 		printf("From %s Mon Sep 17 00:00:00 2001\n", sha1);
+		if (opt->message_id)
+			printf("Message-Id: <%s>\n", opt->message_id);
+		if (opt->ref_message_id)
+			printf("In-Reply-To: <%s>\nReferences: <%s>\n",
+			       opt->ref_message_id, opt->ref_message_id);
 		if (opt->mime_boundary) {
 			static char subject_buffer[1024];
 			static char buffer[1024];

revision.h

@@ -61,6 +61,8 @@ struct rev_info {
 	struct log_info *loginfo;
 	int		nr, total;
 	const char	*mime_boundary;
+	const char	*message_id;
+	const char	*ref_message_id;
 	const char	*add_signoff;
 	const char	*extra_headers;