format-patch: make cover letters always text/plain

When formatting a series of patches using --attach and --cover-letter,
the cover letter lacks the closing MIME boundary, violating RFC 2046.
Certain clients, such as Thunderbird, discard the message body in such a
case.

Since the cover letter is just one part and sending it as
multipart/mixed is not very useful, always emit it as text/plain,
avoiding the boundary problem altogether.

Reported-by: Patrick Hemmer <git@stormcloud9.net>
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/log.c

@@ -1019,7 +1019,7 @@ static void make_cover_letter(struct rev_info *rev, int use_stdout,
 	    open_next_file(NULL, rev->numbered_files ? NULL : "cover-letter", rev, quiet))
 		return;
 
-	log_write_email_headers(rev, head, &pp.after_subject, &need_8bit_cte);
+	log_write_email_headers(rev, head, &pp.after_subject, &need_8bit_cte, 0);
 
 	for (i = 0; !need_8bit_cte && i < nr; i++) {
 		const char *buf = get_commit_buffer(list[i], NULL);