stages/files: add previousReport to result report

This change notices if a report already exists and accordingly
nest the report in previousReport.
related to #1214

internal/exec/stages/files/filesystemEntries.go

@@ -140,20 +140,32 @@ func (s *stage) createResultFile() error {
 	}
 
 	s.Logger.PushPrefix("createResultFile")
+	var prevReport interface{}
+	prevPath, _ := s.JoinPath(distro.ResultFilePath())
+
+	prevData, err := ioutil.ReadFile(prevPath)
+	if err != nil && !os.IsNotExist(err) {
+		return fmt.Errorf("reading previous report: %w", err)
+	}
+	err = json.Unmarshal(prevData, &prevReport)
+	if err != nil {
+		return fmt.Errorf("couldn't unmarshal previous report output: %v", err)
+	}
 	defer s.Logger.PopPrefix()
 
 	bootIDBytes, err := ioutil.ReadFile(distro.BootIDPath())
 	if err != nil {
 		return fmt.Errorf("reading boot ID: %w", err)
 	}
-
 	result := struct {
-		ProvisioningBootID string `json:"provisioningBootID"`
-		ProvisioningDate   string `json:"provisioningDate"`
-		UserConfigProvided bool   `json:"userConfigProvided"`
+		ProvisioningBootID string      `json:"provisioningBootID"`
+		ProvisioningDate   string      `json:"provisioningDate"`
+		UserConfigProvided bool        `json:"userConfigProvided"`
+		PreviousReport     interface{} `json:"previousReport,omitempty"`
 	}{
 		ProvisioningBootID: strings.TrimSpace(string(bootIDBytes)),
 		ProvisioningDate:   time.Now().Format(time.RFC3339),
+		PreviousReport:     prevReport,
 	}
 	for _, config := range s.State.FetchedConfigs {
 		if config.Kind == "user" {