ui, termstatus: Move WrapStdio

Saves some imports. ui still needs to import ui/termstatus from
message.go.

internal/ui/termstatus/stdio_wrapper.go

@@ -0,0 +1,47 @@
+package termstatus
+
+import (
+	"bytes"
+	"io"
+)
+
+// WrapStdio returns line-buffering replacements for os.Stdout and os.Stderr.
+// On Close, the remaining bytes are written, followed by a line break.
+func WrapStdio(term *Terminal) (stdout, stderr io.WriteCloser) {
+	return newLineWriter(term.Print), newLineWriter(term.Error)
+}
+
+type lineWriter struct {
+	buf   bytes.Buffer
+	print func(string)
+}
+
+var _ io.WriteCloser = &lineWriter{}
+
+func newLineWriter(print func(string)) *lineWriter {
+	return &lineWriter{print: print}
+}
+
+func (w *lineWriter) Write(data []byte) (n int, err error) {
+	n, err = w.buf.Write(data)
+	if err != nil {
+		return n, err
+	}
+
+	// look for line breaks
+	buf := w.buf.Bytes()
+	i := bytes.LastIndexByte(buf, '\n')
+	if i != -1 {
+		w.print(string(buf[:i+1]))
+		w.buf.Next(i + 1)
+	}
+
+	return n, err
+}
+
+func (w *lineWriter) Close() error {
+	if w.buf.Len() > 0 {
+		w.print(string(append(w.buf.Bytes(), '\n')))
+	}
+	return nil
+}