mailinfo: support format=flowed

mailinfo: support format=flowed

Add best-effort support for patches sent using format=flowed (RFC 3676).
Remove leading spaces ("unstuff"), remove soft line breaks (indicated
by space + newline), but leave the signature separator (dash dash space
newline) alone.

Warn in git am when encountering a format=flowed patch, because any
trailing spaces would most probably be lost, as the sending MUA is
encouraged to remove them when preparing the email.

Provide a test patch formatted by Mozilla Thunderbird 60 using its
default configuration.  It reuses the contents of the file mailinfo.c
before and after this patch.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>