am: fix patch format detection for Thunderbird "Save As" emails

am: fix patch format detection for Thunderbird "Save As" emails

The patch detection wants to inspect all the headers of a rfc2822 message
and ensure that they look like header fields. The headers are always
separated from the message body with a blank line. When Thunderbird saves
the message the blank line separating the headers from the body includes a
CR. The patch detection is failing because a CRLF doesn't match /^$/. Fix
this by allowing a CR to exist on the separating line.

Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>