trailer: ignore comment lines inside the trailers

Otherwise trailers that are commented out might be
processed. We would also error out if the comment line
char is also a separator.

This means that comments inside a trailer block will
disappear, but that was already the case anyway.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
trailer: display a trailer without its trailing newline

Trailers passed to the parse_trailer() function often have
a trailing newline. When erroring out, we should display
the invalid trailer properly, that means without any
trailing newline.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>