Implement btrfs usage

Implements btrfs usage using a double walking diff and
counting the result. Walking gives the most accurate
count and includes inode usage.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

fs/du_windows.go

@@ -3,6 +3,7 @@
 package fs
 
 import (
+	"context"
 	"os"
 	"path/filepath"
 )
@@ -31,3 +32,29 @@ func diskUsage(roots ...string) (Usage, error) {
 		Size: size,
 	}, nil
 }
+
+func diffUsage(ctx context.Context, a, b string) (Usage, error) {
+	var (
+		size int64
+	)
+
+	if err := Changes(ctx, a, b, func(kind ChangeKind, _ string, fi os.FileInfo, err error) error {
+		if err != nil {
+			return err
+		}
+
+		if kind == ChangeKindAdd || kind == ChangeKindModify {
+			size += fi.Size()
+
+			return nil
+
+		}
+		return nil
+	}); err != nil {
+		return Usage{}, err
+	}
+
+	return Usage{
+		Size: size,
+	}, nil
+}