Add option to enable threading headers

Add a --thread option to enable generation of In-Reply-To and References
headers, used to make the second and subsequent mails appear as replies to the
first.
Signed-off-by: NJosh Triplett <josh@freedesktop.org>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Documentation/git-format-patch.txt

@@ -9,7 +9,7 @@ git-format-patch - Prepare patches for e-mail submission
 SYNOPSIS
 --------
 [verse]
-'git-format-patch' [-n | -k] [-o <dir> | --stdout] [--attach]
+'git-format-patch' [-n | -k] [-o <dir> | --stdout] [--attach] [--thread]
 	           [-s | --signoff] [--diff-options] [--start-number <n>]
 		   <since>[..<until>]
 
@@ -35,6 +35,10 @@ they are created in the current working directory.
 If -n is specified, instead of "[PATCH] Subject", the first line
 is formatted as "[PATCH n/m] Subject".
 
+If given --thread, git-format-patch will generate In-Reply-To and
+References headers to make the second and subsequent patch mails appear
+as replies to the first mail; this also generates a Message-Id header to
+reference.
 
 OPTIONS
 -------
@@ -63,6 +67,10 @@ OPTIONS
 --attach::
 	Create attachments instead of inlining patches.
 
+--thread::
+	Add In-Reply-To and References headers to make the second and
+	subsequent mails appear as replies to the first.  Also generates
+	the Message-Id header to reference.
 
 CONFIGURATION
 -------------

builtin-log.c

@@ -248,6 +248,7 @@ int cmd_format_patch(int argc, const char **argv, char **envp)
 	int start_number = -1;
 	int keep_subject = 0;
 	int ignore_if_in_upstream = 0;
+	int thread = 0;
 	struct diff_options patch_id_opts;
 	char *add_signoff = NULL;
 	char message_id[1024];
@@ -317,6 +318,8 @@ int cmd_format_patch(int argc, const char **argv, char **envp)
 			rev.mime_boundary = argv[i] + 9;
 		else if (!strcmp(argv[i], "--ignore-if-in-upstream"))
 			ignore_if_in_upstream = 1;
+		else if (!strcmp(argv[i], "--thread"))
+			thread = 1;
 		else
 			argv[j++] = argv[i];
 	}
@@ -379,15 +382,17 @@ int cmd_format_patch(int argc, const char **argv, char **envp)
 		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;
+		if (thread) {
+			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;
 		}
-		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);