commit re-encoding: fix confusion between no and default conversion.

Telling the git-log family not to do any character conversion is
done with --encoding=none, which sets log_output_encoding to an
empty string.  However, logmsg_reencode() confused this with
log_output_encoding and commit_encoding set to NULL.  The latter
means we should use the default encoding (i.e. utf-8).

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

commit.c

@@ -633,6 +633,8 @@ static char *logmsg_reencode(const struct commit *commit)
 				 : git_commit_encoding);
 
 	if (!output_encoding)
+		output_encoding = "utf-8";
+	else if (!*output_encoding)
 		return NULL;
 	encoding = get_header(commit, "encoding");
 	if (!encoding || !strcmp(encoding, output_encoding)) {