format-patch: fix ignored encode_email_headers for cover letter

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>