Bug: format-patch MIME boundary not added to cover letter when attach enabled

[Patrick Hemmer <git@stormcloud9.net>]

When you use `git format-patch --cover-letter --attach`, the cover
letter does not have the trailing MIME boundary. RFC2046 states that the
last part must be followed by a closing boundary. This causes some email
clients (Thunderbird in my case) to discard the message body.
This is experienced with git 2.16.3.

For example:

$ git format-patch --cover-letter --attach --root -o /tmp/out
/tmp/out/0000-cover-letter.patch
/tmp/out/0001-hello-world.patch

$ cat /tmp/out/0000-cover-letter.patch
From a25ac88e6216131e8b000335d32bb99d4e5185ac Mon Sep 17 00:00:00 2001
From: Patrick Hemmer <git@stormcloud9.net>
Date: Sun, 29 Apr 2018 21:26:45 -0400
Subject: [PATCH 0/1] *** SUBJECT HERE ***
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="------------2.16.3"

This is a multi-part message in MIME format.
--------------2.16.3
Content-Type: text/plain; charset=UTF-8; format=fixed
Content-Transfer-Encoding: 8bit


*** BLURB HERE ***

Patrick Hemmer (1):
  hello world

-- 
2.16.3