send-email: add --[no-]xmailer option

Add --[no-]xmailer that allows a user to disable adding the 'X-Mailer:'
header to the email being sent.

Signed-off-by: Luis Henriques <henrix@camandro.org>
Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -2260,6 +2260,7 @@ sendemail.smtpserveroption::
 sendemail.smtpuser::
 sendemail.thread::
 sendemail.validate::
+sendemail.xmailer::
 	See linkgit:git-send-email[1] for description.
 
 sendemail.signedoffcc::

Documentation/git-send-email.txt

@@ -131,6 +131,11 @@ Note that no attempts whatsoever are made to validate the encoding.
 	Specify encoding of compose message. Default is the value of the
 	'sendemail.composeencoding'; if that is unspecified, UTF-8 is assumed.
 
+--xmailer::
+--no-xmailer::
+	Add (or prevent adding) the "X-Mailer:" header.  By default,
+	the header is added, but it can be turned off by setting the
+	`sendemail.xmailer` configuration variable to `false`.
 
 Sending
 ~~~~~~~

git-send-email.perl

@@ -54,6 +54,7 @@ git send-email [options] <file | directory | rev-list options >
     --[no-]bcc              <str>  * Email Bcc:
     --subject               <str>  * Email "Subject:"
     --in-reply-to           <str>  * Email "In-Reply-To:"
+    --[no-]xmailer                 * Add "X-Mailer:" header (default).
     --[no-]annotate                * Review each patch that will be sent in an editor.
     --compose                      * Open an editor for introduction.
     --compose-encoding      <str>  * Encoding to assume for introduction.
@@ -148,7 +149,7 @@ my $auth;
 # Variables we fill in automatically, or via prompting:
 my (@to,$no_to,@initial_to,@cc,$no_cc,@initial_cc,@bcclist,$no_bcc,@xh,
 	$initial_reply_to,$initial_subject,@files,
-	$author,$sender,$smtp_authpass,$annotate,$compose,$time);
+	$author,$sender,$smtp_authpass,$annotate,$use_xmailer,$compose,$time);
 
 my $envelope_sender;
 
@@ -219,7 +220,8 @@ my %config_bool_settings = (
     "signedoffcc" => [\$signed_off_by_cc, undef],      # Deprecated
     "validate" => [\$validate, 1],
     "multiedit" => [\$multiedit, undef],
-    "annotate" => [\$annotate, undef]
+    "annotate" => [\$annotate, undef],
+    "xmailer" => [\$use_xmailer, 1]
 );
 
 my %config_settings = (
@@ -318,6 +320,7 @@ my $rc = GetOptions("h" => \$help,
 		    "8bit-encoding=s" => \$auto_8bit_encoding,
 		    "compose-encoding=s" => \$compose_encoding,
 		    "force" => \$force,
+		    "xmailer!" => \$use_xmailer,
 	 );
 
 usage() if $help;
@@ -1163,8 +1166,10 @@ To: $to${ccline}
 Subject: $subject
 Date: $date
 Message-Id: $message_id
-X-Mailer: git-send-email $gitversion
 ";
+	if ($use_xmailer) {
+		$header .= "X-Mailer: git-send-email $gitversion\n";
+	}
 	if ($reply_to) {
 
 		$header .= "In-Reply-To: $reply_to\n";