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.go

@@ -1,5 +1,7 @@
 package fs
 
+import "context"
+
 // Usage of disk information
 type Usage struct {
 	Inodes int64
@@ -11,3 +13,10 @@ type Usage struct {
 func DiskUsage(roots ...string) (Usage, error) {
 	return diskUsage(roots...)
 }
+
+// DiffUsage counts the numbers of inodes and disk usage in the
+// diff between the 2 directories. The first path is intended
+// as the base directory and the second as the changed directory.
+func DiffUsage(ctx context.Context, a, b string) (Usage, error) {
+	return diffUsage(ctx, a, b)
+}