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>

1 files changed, 2 insertions, 0 deletions

diff --git a/mailinfo.h b/mailinfo.h
index 04a2535..67af28d 100644
--- a/mailinfo.h
+++ b/mailinfo.h
@@ -20,6 +20,8 @@ struct mailinfo {
 	struct strbuf *content[MAX_BOUNDARIES];
 	struct strbuf **content_top;
 	struct strbuf charset;
+	unsigned int format_flowed:1;
+	unsigned int delsp:1;
 	char *message_id;
 	enum  {
 		TE_DONTCARE, TE_QP, TE_BASE64