format-patch: fix ignored encode_email_headers for cover letter

When writing the cover letter, the encode_email_headers option was
ignored. That is, UTF-8 subject lines and email addresses were
written out as-is, without any Q-encoding, even if
--encode-email-headers was passed on the command line.

This is due to encode_email_headers not being copied over from
struct rev_info to struct pretty_print_context. Fix that and add
a test.

Signed-off-by: Simon Ser <contact@emersion.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/log.c

@@ -1364,6 +1364,7 @@ static void make_cover_letter(struct rev_info *rev, int use_separate_file,
 	pp.date_mode.type = DATE_RFC2822;
 	pp.rev = rev;
 	pp.print_email_subject = 1;
+	pp.encode_email_headers = rev->encode_email_headers;
 	pp_user_info(&pp, NULL, &sb, committer, encoding);
 	prepare_cover_text(&pp, description_file, branch_name, &sb,
 			   encoding, need_8bit_cte);

t/t4014-format-patch.sh

@@ -1906,6 +1906,16 @@ body" &&
 	grep "^body$" actual
 '
 
+test_expect_success 'cover letter with --cover-from-description subject (UTF-8 subject line)' '
+	test_config branch.rebuild-1.description "Café?
+
+body" &&
+	git checkout rebuild-1 &&
+	git format-patch --stdout --cover-letter --cover-from-description subject --encode-email-headers main >actual &&
+	grep "^Subject: \[PATCH 0/2\] =?UTF-8?q?Caf=C3=A9=3F?=$" actual &&
+	! grep "Café" actual
+'
+
 test_expect_success 'cover letter with format.coverFromDescription = auto (short subject line)' '
 	test_config branch.rebuild-1.description "config subject