lock: add Refresh method

lock_test.go

@@ -195,3 +195,33 @@ func TestRemoveAllLocks(t *testing.T) {
 	Assert(t, lockExists(repo, t, id3) == false,
 		"lock still exists after RemoveAllLocks was called")
 }
+
+func TestLockRefresh(t *testing.T) {
+	repo := SetupRepo()
+	defer TeardownRepo(repo)
+
+	lock, err := restic.NewLock(repo)
+	OK(t, err)
+
+	var lockID backend.ID
+	for id := range repo.List(backend.Lock, nil) {
+		if lockID != nil {
+			t.Error("more than one lock found")
+		}
+		lockID = id
+	}
+
+	OK(t, lock.Refresh())
+
+	var lockID2 backend.ID
+	for id := range repo.List(backend.Lock, nil) {
+		if lockID2 != nil {
+			t.Error("more than one lock found")
+		}
+		lockID2 = id
+	}
+
+	Assert(t, !lockID.Equal(lockID2),
+		"expected a new ID after lock refresh, got the same")
+	OK(t, lock.Unlock())
+}