t9001-send-email: refactor header variable fields replacement

Create a function which replaces Date, Message-Id and
X-Mailer lines generated by git-send-email by a specific string:

Date:.*$       -> Date: DATE-STRING
Message-Id:.*$ -> Message-Id: MESSAGE-ID-STRING
X-Mailer:.*$   -> X-Mailer: X-MAILER-STRING
Signed-off-by: Remi Lespinet <[email protected]>
Signed-off-by: Matthieu Moy <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Awaryn

t/t9001-send-email.sh

@@ -522,17 +522,20 @@ Result: OK
 EOF
 "
 
+replace_variable_fields () {
+	sed	-e "s/^\(Date:\).*/\1 DATE-STRING/" \
+		-e "s/^\(Message-Id:\).*/\1 MESSAGE-ID-STRING/" \
+		-e "s/^\(X-Mailer:\).*/\1 X-MAILER-STRING/"
+}
+
 test_suppression () {
 	git send-email \
 		--dry-run \
 		--suppress-cc=$1 ${2+"--suppress-cc=$2"} \
 		--from="Example <[email protected]>" \
 		[email protected] \
 		--smtp-server relay.example.com \
-		$patches |
-	sed	-e "s/^\(Date:\).*/\1 DATE-STRING/" \
-		-e "s/^\(Message-Id:\).*/\1 MESSAGE-ID-STRING/" \
-		-e "s/^\(X-Mailer:\).*/\1 X-MAILER-STRING/" \
+		$patches | replace_variable_fields \
 		>actual-suppress-$1${2+"-$2"} &&
 	test_cmp expected-suppress-$1${2+"-$2"} actual-suppress-$1${2+"-$2"}
 }