mailinfo: correctly handle multiline 'Subject:' header

When native language (RU) is in use, subject header usually contains several
parts, e.g.

Subject: [Navy-patches] [PATCH]
	=?utf-8?b?0JjQt9C80LXQvdGR0L0g0YHQv9C40YHQvtC6INC/0LA=?=
	=?utf-8?b?0LrQtdGC0L7QsiDQvdC10L7QsdGF0L7QtNC40LzRi9GFINC00LvRjyA=?=
	=?utf-8?b?0YHQsdC+0YDQutC4?=

This exposes several bugs in builtin-mailinfo.c:

1. decode_b_segment: do not append explicit NUL -- explicit NUL was preventing
   correct header construction on parts concatenation via strbuf_addbuf in
   decode_header_bq.  Fixes:

-Subject: Изменён список пакетов необходимых для сборки
+Subject: Изменён список па

Then

2. Do not emit '\n' between "encoded-word" where RFC2046 says that linear
   white space between them are ignored when displaying.  Fixes:

-Subject: Изменён список пакетов необходимых для сборки
+Subject: Изменён список па кетов необходимых для сборки

Signed-off-by: Kirill Smelkov <kirr@mns.spb.ru>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 19 insertions, 8 deletions

diff --git a/builtin-mailinfo.c b/builtin-mailinfo.c
index e890f7a6d1..fcb32c9818 100644
--- a/builtin-mailinfo.c
+++ b/builtin-mailinfo.c
@@ -430,13 +430,6 @@ static struct strbuf *decode_b_segment(const struct strbuf *b_seg)
 			c -= 'a' - 26;
 		else if ('0' <= c && c <= '9')
 			c -= '0' - 52;
-		else if (c == '=') {
-			/* padding is almost like (c == 0), except we do
-			 * not output NUL resulting only from it;
-			 * for now we just trust the data.
-			 */
-			c = 0;
-		}
 		else
 			continue; /* garbage */
 		switch (pos++) {
@@ -514,7 +507,25 @@ static int decode_header_bq(struct strbuf *it)
 		rfc2047 = 1;
 
 		if (in != ep) {
-			strbuf_add(&outbuf, in, ep - in);
+			/*
+			 * We are about to process an encoded-word
+			 * that begins at ep, but there is something
+			 * before the encoded word.
+			 */
+			char *scan;
+			for (scan = in; scan < ep; scan++)
+				if (!isspace(*scan))
+					break;
+
+			if (scan != ep || in == it->buf) {
+				/*
+				 * We should not lose that "something",
+				 * unless we have just processed an
+				 * encoded-word, and there is only LWS
+				 * before the one we are about to process.
+				 */
+				strbuf_add(&outbuf, in, ep - in);
+			}
 			in = ep;
 		}
 		/* E.g.