git: teach *PktlineWriter.Flush() to respond to nil receiver

git/pkt_line_writer.go

@@ -87,6 +87,10 @@ func (w *PktlineWriter) Write(p []byte) (int, error) {
 // writes the pkt-line's FLUSH packet, to signal that it is done writing this
 // chunk of data.
 func (w *PktlineWriter) Flush() error {
+	if w == nil {
+		return nil
+	}
+
 	if _, err := w.flush(); err != nil {
 		return err
 	}

git/pkt_line_writer_test.go

@@ -81,6 +81,10 @@ func TestPktlineWriterWritesMultiplePacketsGreaterThanMaxPacketLength(t *testing
 	assertPacketRead(t, pl, nil)
 }
 
+func TestPktlineWriterAllowsFlushesOnNil(t *testing.T) {
+	assert.NoError(t, (*PktlineWriter)(nil).Flush())
+}
+
 func TestPktlineWriterDoesntWrapItself(t *testing.T) {
 	itself := &PktlineWriter{}
 	nw := NewPktlineWriter(itself, 0)