Don't print reference count for json v1

The reference count is still displayed when the -j flag is passed to
git-sizer.
```
git-sizer master~50^{tree} 'HEAD@{1.week.ago}' -v -j
[TRUNCATED_OUTPUT]
    "reference_count": 15,
    "reference_groups": {
        "ignored": 15
    },
[TRUNCATED_OUTPUT]
```

We don't want to print the reference count for json v1, when explicit
root are not provided git-sizer

Author: elhmn

git-sizer.go

@@ -352,7 +352,7 @@ func mainImplementation(ctx context.Context, stdout, stderr io.Writer, args []st
 		var err error
 		switch jsonVersion {
 		case 1:
-			j, err = json.MarshalIndent(historySize, "", "    ")
+			j, err = json.MarshalIndent(historySize.JsonV1Format(&formatOptions), "", "    ")
 		case 2:
 			j, err = historySize.JSON(rg.Groups(), threshold, nameStyle, formatOptions)
 		default:

sizes/output.go

@@ -41,6 +41,19 @@ func (s TagSize) String() string {
 	return fmt.Sprintf("tag_depth=%d", s.TagDepth)
 }
 
+func (s *HistorySize) JsonV1Format(opts *FormatOptions) *HistorySize {
+	if opts == nil {
+		return s
+	}
+
+	if opts.WithoutReferenceCount {
+		s.ReferenceCount = 0
+		s.ReferenceGroups = nil
+	}
+
+	return s
+}
+
 func (s *HistorySize) String() string {
 	return fmt.Sprintf(
 		"unique_commit_count=%d, unique_commit_count = %d, max_commit_size = %d, "+

sizes/sizes.go

@@ -158,11 +158,11 @@ type HistorySize struct {
 	// The number of references analyzed. Note that we don't eliminate
 	// duplicates if the user passes the same reference more than
 	// once.
-	ReferenceCount counts.Count32 `json:"reference_count"`
+	ReferenceCount counts.Count32 `json:"reference_count,omitempty"`
 
 	// ReferenceGroups keeps track of how many references in each
 	// reference group were scanned.
-	ReferenceGroups map[RefGroupSymbol]*counts.Count32 `json:"reference_groups"`
+	ReferenceGroups map[RefGroupSymbol]*counts.Count32 `json:"reference_groups,omitempty"`
 
 	// The maximum TreeSize in the analyzed history (where each
 	// attribute is maximized separately).