cmd: fix cursor flickering in progress bar

The previous commit fixed flickering in the progress bar itself. Cursor
flickering is harder to address.

Cursor flickering could be fixed by hiding the cursor altogether while
the progress bar is displayed. The downside of this is that if the
program is killed in such a way that it can't clean up its state, it
would leave the cursor invisible.

Instead, this commit introduces an output buffer. All of the escape
codes and content for a single progress update are written to a buffer,
which is then flushed to the terminal all at once. This significantly
decreases the time during which the terminal has seen the cursor-hiding
code but has not yet seen the cursor-showing code, thus minimizing (but
not 100% eliminating) cursor flickering.

For more context, see:
https://gitlab.gnome.org/GNOME/vte/-/issues/2837#note_2269501

progress/progress.go

 package progress
 
 import (
+	"bytes"
 	"fmt"
 	"io"
 	"sync"
@@ -12,8 +13,9 @@ type State interface {
 }
 
 type Progress struct {
-	mu sync.Mutex
-	w  io.Writer
+	mu  sync.Mutex
+	w   io.Writer
+	buf bytes.Buffer
 
 	pos int
 
@@ -81,21 +83,26 @@ func (p *Progress) render() {
 	p.mu.Lock()
 	defer p.mu.Unlock()
 
-	fmt.Fprint(p.w, "\033[?25l")
-	defer fmt.Fprint(p.w, "\033[?25h")
+	// buffer the terminal update to minimize cursor flickering
+	// https://gitlab.gnome.org/GNOME/vte/-/issues/2837#note_2269501
+	p.buf.Reset()
+	defer p.buf.WriteTo(p.w)
+
+	fmt.Fprint(&p.buf, "\033[?25l")
+	defer fmt.Fprint(&p.buf, "\033[?25h")
 
 	// move the cursor back to the beginning
 	for range p.pos - 1 {
-		fmt.Fprint(p.w, "\033[A")
+		fmt.Fprint(&p.buf, "\033[A")
 	}
-	fmt.Fprint(p.w, "\033[1G")
+	fmt.Fprint(&p.buf, "\033[1G")
 
 	// render progress lines
 	for i, state := range p.states {
-		fmt.Fprint(p.w, state.String())
-		fmt.Fprintf(p.w, "\033[K")
+		fmt.Fprint(&p.buf, state.String())
+		fmt.Fprintf(&p.buf, "\033[K")
 		if i < len(p.states)-1 {
-			fmt.Fprint(p.w, "\n")
+			fmt.Fprint(&p.buf, "\n")
 		}
 	}