format-patch: track several references

Currently, format-patch can only track a single reference (the
In-Reply-To:) for each mail.  To ensure proper threading, we should
list all known references for every mail.

Change the rev_info.ref_message_id field to a string_list, so that we
can append references at will, and change the output formatting
routines to print all of them in the References: header.  The last
entry in the list is implicitly assumed to be the In-Reply-To:, which
gives output consistent with RFC 2822:

   The "References:" field will contain the contents of the parent's
   "References:" field (if any) followed by the contents of the
   parent's "Message-ID:" field (if any).

Note that this is just preparatory work; nothing uses it yet, so all
"References:" fields in the output are still only one deep.

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

builtin-log.c

@@ -17,6 +17,7 @@
 #include "run-command.h"
 #include "shortlog.h"
 #include "remote.h"
+#include "string-list.h"
 
 /* Set a default date-time format for git log ("log.date" config variable) */
 static const char *default_date_mode = NULL;
@@ -1011,8 +1012,12 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 		numbered = 1;
 	if (numbered)
 		rev.total = total + start_number - 1;
-	if (in_reply_to)
-		rev.ref_message_id = clean_message_id(in_reply_to);
+	if (in_reply_to || thread || cover_letter)
+		rev.ref_message_ids = xcalloc(1, sizeof(struct string_list));
+	if (in_reply_to) {
+		const char *msgid = clean_message_id(in_reply_to);
+		string_list_append(msgid, rev.ref_message_ids);
+	}
 	if (cover_letter) {
 		if (thread)
 			gen_message_id(&rev, "cover");
@@ -1036,10 +1041,11 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 				 * otherwise, make everything a reply
 				 * to that.
 				 */
-				if (rev.ref_message_id)
+				if (rev.ref_message_ids->nr > 0)
 					free(rev.message_id);
 				else
-					rev.ref_message_id = rev.message_id;
+					string_list_append(rev.message_id,
+							   rev.ref_message_ids);
 			}
 			gen_message_id(&rev, sha1_to_hex(commit->object.sha1));
 		}

log-tree.c

@@ -6,6 +6,7 @@
 #include "log-tree.h"
 #include "reflog-walk.h"
 #include "refs.h"
+#include "string-list.h"
 
 struct decoration name_decoration = { "object names" };
 
@@ -211,9 +212,13 @@ void log_write_email_headers(struct rev_info *opt, const char *name,
 		printf("Message-Id: <%s>\n", opt->message_id);
 		graph_show_oneline(opt->graph);
 	}
-	if (opt->ref_message_id) {
-		printf("In-Reply-To: <%s>\nReferences: <%s>\n",
-		       opt->ref_message_id, opt->ref_message_id);
+	if (opt->ref_message_ids && opt->ref_message_ids->nr > 0) {
+		int i, n;
+		n = opt->ref_message_ids->nr;
+		printf("In-Reply-To: <%s>\n", opt->ref_message_ids->items[n-1].string);
+		for (i = 0; i < n; i++)
+			printf("%s<%s>\n", (i > 0 ? "\t" : "References: "),
+			       opt->ref_message_ids->items[i].string);
 		graph_show_oneline(opt->graph);
 	}
 	if (opt->mime_boundary) {

revision.h

@@ -89,7 +89,7 @@ struct rev_info {
 	int		nr, total;
 	const char	*mime_boundary;
 	char		*message_id;
-	const char	*ref_message_id;
+	struct string_list *ref_message_ids;
 	const char	*add_signoff;
 	const char	*extra_headers;
 	const char	*log_reencode;