t4014: "no-add-headers" is actually called "no-add-header"

Since c426003 (format-patch: add --no-cc, --no-to, and
--no-add-headers, 2010-03-07) the tests have checked for an option
called --no-add-headers introduced by letting the user negate
--add-header.

However, the parseopt machinery does not automatically pluralize
anything, so it is in fact called --no-add-header.

Since the option never worked, is not documented anywhere, and
implementing an actual --no-add-headers would lead to silly code
complications, we just adapt the test to the code.

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

t/t4014-format-patch.sh

@@ -219,11 +219,11 @@ test_expect_success '--no-cc overrides config.cc' '
 	! grep "^Cc: C. E. Cipient <rcipient@example.com>\$" patch12
 '
 
-test_expect_failure '--no-add-headers overrides config.headers' '
+test_expect_success '--no-add-header overrides config.headers' '
 
 	git config --replace-all format.headers \
 		"Header1: B. E. Cipient <rcipient@example.com>" &&
-	git format-patch --no-add-headers --stdout master..side |
+	git format-patch --no-add-header --stdout master..side |
 	sed -e "/^\$/q" >patch13 &&
 	check_patch patch13 &&
 	! grep "^Header1: B. E. Cipient <rcipient@example.com>\$" patch13