fix: bug where hard links could be double counted

When running:
dust dir_a dir_b

if a file was hard linked in both dir_a and dir_b it would be double
counted.

This fix resolves this by keeping the shared hashmap around between runs
for the second and subsequent arguments.
https://github.com/bootandy/dust/issues/282

src/dir_walker.rs

@@ -31,12 +31,13 @@ pub struct WalkData<'a> {
 pub fn walk_it(dirs: HashSet<PathBuf>, walk_data: WalkData) -> (Vec<Node>, bool) {
     let permissions_flag = AtomicBool::new(false);
 
+    let mut inodes = HashSet::new();
     let top_level_nodes: Vec<_> = dirs
         .into_iter()
         .filter_map(|d| {
             clean_inodes(
                 walk(d, &permissions_flag, &walk_data, 0)?,
-                &mut HashSet::new(),
+                &mut inodes,
                 walk_data.use_apparent_size,
             )
         })