Control progress rate with RESTIC_PROGRESS_FPS env

Add a RESTIC_PROGRESS_FPS environment variable to limit the interval
at which the progress indicator updates (allowed values: 1-60).

The default rate of 60 FPS can cause high terminal CPU load on some
systems, like iTerm2 on macOS with font anti-aliasing enabled.

Usage:

    RESTIC_PROGRESS_FPS=1 restic ...
    RESTIC_PROGRESS_FPS=60 restic ...

internal/restic/progress.go

@@ -3,17 +3,31 @@ package restic
 import (
 	"fmt"
 	"os"
+	"strconv"
 	"sync"
 	"time"
 
 	"golang.org/x/crypto/ssh/terminal"
 )
 
-const minTickerTime = time.Second / 60
+// minTickerTime limits how often the progress ticker is updated. It can be
+// overridden using the RESTIC_PROGRESS_FPS (frames per second) environment
+// variable.
+var minTickerTime = time.Second / 60
 
 var isTerminal = terminal.IsTerminal(int(os.Stdout.Fd()))
 var forceUpdateProgress = make(chan bool)
 
+func init() {
+	fps, err := strconv.ParseInt(os.Getenv("RESTIC_PROGRESS_FPS"), 10, 64)
+	if err == nil && fps >= 1 {
+		if fps > 60 {
+			fps = 60
+		}
+		minTickerTime = time.Second / time.Duration(fps)
+	}
+}
+
 // Progress reports progress on an operation.
 type Progress struct {
 	OnStart  func()