format-patch: add --reroll-count=$N option

The --reroll-count=$N option, when given a positive integer:

 - Adds " v$N" to the subject prefix specified.  As the default
   subject prefix string is "PATCH", --reroll-count=2 makes it
   "PATCH v2".

 - Prefixes "v$N-" to the names used for output files.  The cover
   letter, whose name is usually 0000-cover-letter.patch, becomes
   v2-0000-cover-letter.patch when given --reroll-count=2.

This allows users to use the same --output-directory for multiple
iterations of the same series, without letting the output for a
newer round overwrite output files from the earlier rounds.  The
user can incorporate materials from earlier rounds to update the
newly minted iteration, and use "send-email v2-*.patch" to send out
the patches belonging to the second iteration easily.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/log.c

@@ -1061,6 +1061,7 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 	struct strbuf buf = STRBUF_INIT;
 	int use_patch_format = 0;
 	int quiet = 0;
+	int reroll_count = -1;
 	char *branch_name = NULL;
 	const struct option builtin_format_patch_options[] = {
 		{ OPTION_CALLBACK, 'n', "numbered", &numbered, NULL,
@@ -1080,6 +1081,8 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 			    N_("use <sfx> instead of '.patch'")),
 		OPT_INTEGER(0, "start-number", &start_number,
 			    N_("start numbering patches at <n> instead of 1")),
+		OPT_INTEGER(0, "reroll-count", &reroll_count,
+			    N_("mark the series as Nth re-roll")),
 		{ OPTION_CALLBACK, 0, "subject-prefix", &rev, N_("prefix"),
 			    N_("Use [<prefix>] instead of [PATCH]"),
 			    PARSE_OPT_NONEG, subject_prefix_callback },
@@ -1152,6 +1155,14 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 			     PARSE_OPT_KEEP_ARGV0 | PARSE_OPT_KEEP_UNKNOWN |
 			     PARSE_OPT_KEEP_DASHDASH);
 
+	if (0 < reroll_count) {
+		struct strbuf sprefix = STRBUF_INIT;
+		strbuf_addf(&sprefix, "%s v%d",
+			    rev.subject_prefix, reroll_count);
+		rev.reroll_count = reroll_count;
+		rev.subject_prefix = strbuf_detach(&sprefix, NULL);
+	}
+
 	if (do_signoff) {
 		const char *committer;
 		const char *endpos;

log-tree.c

@@ -308,6 +308,8 @@ void fmt_output_subject(struct strbuf *filename,
 	int start_len = filename->len;
 	int max_len = start_len + FORMAT_PATCH_NAME_MAX - (strlen(suffix) + 1);
 
+	if (0 < info->reroll_count)
+		strbuf_addf(filename, "v%d-", info->reroll_count);
 	strbuf_addf(filename, "%04d-%s", nr, subject);
 
 	if (max_len < filename->len)

revision.h

@@ -134,6 +134,7 @@ struct rev_info {
 	const char	*mime_boundary;
 	const char	*patch_suffix;
 	int		numbered_files;
+	int		reroll_count;
 	char		*message_id;
 	struct string_list *ref_message_ids;
 	const char	*add_signoff;