cmd: fix progress bar flickering

Previous code cleared the display before writing new content, creating a
window where the terminal could (and in some cases did) render empty lines.

Instead, we now write new content over the old content, only clearing
the trailing end of lines for cases where the new line is shorter.

Fixes #1664

progress/progress.go

@@ -84,17 +84,16 @@ func (p *Progress) render() {
 	fmt.Fprint(p.w, "\033[?25l")
 	defer fmt.Fprint(p.w, "\033[?25h")
 
-	// clear already rendered progress lines
-	for i := range p.pos {
-		if i > 0 {
-			fmt.Fprint(p.w, "\033[A")
-		}
-		fmt.Fprint(p.w, "\033[2K\033[1G")
+	// move the cursor back to the beginning
+	for range p.pos - 1 {
+		fmt.Fprint(p.w, "\033[A")
 	}
+	fmt.Fprint(p.w, "\033[1G")
 
 	// render progress lines
 	for i, state := range p.states {
 		fmt.Fprint(p.w, state.String())
+		fmt.Fprintf(p.w, "\033[K")
 		if i < len(p.states)-1 {
 			fmt.Fprint(p.w, "\n")
 		}