retry: limit retries based on elapsed time not count

Depending on how long an operation takes to fail, the total retry
duration can currently vary between 1.5 and 15 minutes. In particular
for temporarily interrupted network connections, the former timeout is
too short. Thus always use a limit of 15 minutes.

cmd/restic/global.go

@@ -425,7 +425,7 @@ func OpenRepository(ctx context.Context, opts GlobalOptions) (*repository.Reposi
 	success := func(msg string, retries int) {
 		Warnf("%v operation successful after %d retries\n", msg, retries)
 	}
-	be = retry.New(be, 10, report, success)
+	be = retry.New(be, 15*time.Minute, report, success)
 
 	// wrap backend if a test specified a hook
 	if opts.backendTestHook != nil {