format-patch: document and test --reroll-count

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

Documentation/git-format-patch.txt

@@ -18,7 +18,7 @@ SYNOPSIS
 		   [--start-number <n>] [--numbered-files]
 		   [--in-reply-to=Message-Id] [--suffix=.<sfx>]
 		   [--ignore-if-in-upstream]
-		   [--subject-prefix=Subject-Prefix]
+		   [--subject-prefix=Subject-Prefix] [--reroll-count <n>]
 		   [--to=<email>] [--cc=<email>]
 		   [--cover-letter] [--quiet]
 		   [<common diff options>]
@@ -166,6 +166,14 @@ will want to ensure that threading is disabled for `git send-email`.
 	allows for useful naming of a patch series, and can be
 	combined with the `--numbered` option.
 
+--reroll-count=<n>::
+	Mark the series as the <n>-th iteration of the topic. The
+	output filenames have `v<n>` pretended to them, and the
+	subject prefix ("PATCH" by default, but configurable via the
+	`--subject-prefix` option) has ` v<n>` appended to it.  E.g.
+	`--reroll-count=4` may produce `v4-0001-add-makefile.patch`
+	file that has "Subject: [PATCH v4 1/20] Add makefile" in it.
+
 --to=<email>::
 	Add a `To:` header to the email headers. This is in addition
 	to any configured headers, and may be used multiple times.

t/t4014-format-patch.sh

@@ -237,6 +237,14 @@ test_expect_success 'multiple files' '
 	ls patches/0001-Side-changes-1.patch patches/0002-Side-changes-2.patch patches/0003-Side-changes-3-with-n-backslash-n-in-it.patch
 '
 
+test_expect_success 'reroll count' '
+	rm -fr patches &&
+	git format-patch -o patches --cover-letter --reroll-count 4 master..side >list &&
+	! grep -v "^patches/v4-000[0-3]-" list &&
+	sed -n -e "/^Subject: /p" $(cat list) >subjects &&
+	! grep -v "^Subject: \[PATCH v4 [0-3]/3\] " subjects
+'
+
 check_threading () {
 	expect="$1" &&
 	shift &&