tests: Add tests for missing format-patch long options

Exercise format-patch's --signoff, --in-reply-to and --start-number long
options.

Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t4021-format-patch-numbered.sh b/t/t4021-format-patch-numbered.sh
index 390af23..3c27f0d 100755
--- a/t/t4021-format-patch-numbered.sh
+++ b/t/t4021-format-patch-numbered.sh
@@ -108,4 +108,10 @@
 
 '
 
+test_expect_success '--start-number && --numbered' '
+
+	git format-patch --start-number 3 --numbered --stdout HEAD~1 > patch8 &&
+	grep "^Subject: \[PATCH 3/3\]" patch8
+'
+
 test_done