provisioner/shell: comment on why we return right away if advance == 0

/cc @mwhooker

provisioner/shell/unix_reader.go

@@ -55,7 +55,11 @@ func (r *UnixReader) Read(p []byte) (n int, err error) {
 func scanUnixLine(data []byte, atEOF bool) (advance int, token []byte, err error) {
 	advance, token, err = bufio.ScanLines(data, atEOF)
 	if advance == 0 {
+		// If we reached the end of a line without a newline, then
+		// just return as it is. Otherwise the Scanner will keep trying
+		// to scan, blocking forever.
 		return
 	}
+
 	return advance, append(token, '\n'), err
 }