commit: fix pretty-printing of messages with "\nencoding "

The function replace_encoding_header is given the whole
commit buffer, including the commit message. When looking
for the encoding header, if none was found in the header, it
would locate any line in the commit message matching
"\nencoding " and remove it.

Instead, we now make sure to search only to the end of the
header.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>

commit.c

@@ -644,6 +644,7 @@ static char *get_header(const struct commit *commit, const char *key)
 static char *replace_encoding_header(char *buf, char *encoding)
 {
 	char *encoding_header = strstr(buf, "\nencoding ");
+	char *header_end = strstr(buf, "\n\n");
 	char *end_of_encoding_header;
 	int encoding_header_pos;
 	int encoding_header_len;
@@ -651,8 +652,10 @@ static char *replace_encoding_header(char *buf, char *encoding)
 	int need_len;
 	int buflen = strlen(buf) + 1;
 
-	if (!encoding_header)
-		return buf; /* should not happen but be defensive */
+	if (!header_end)
+		header_end = buf + buflen;
+	if (!encoding_header || encoding_header >= header_end)
+		return buf;
 	encoding_header++;
 	end_of_encoding_header = strchr(encoding_header, '\n');
 	if (!end_of_encoding_header)