dirtree: fix tests with -fast-list

In commit

da404dc0f2 sync,copy: Fix --fast-list --create-empty-src-dirs and --exclude

The fix caused DirTree.AddDir to be called with the root directory.
This in turn caused a spurious directory entry in the DirTree which
caused tests with the -fast-list flag to fail with directory not found
errors.

fs/dirtree/dirtree.go

@@ -40,9 +40,12 @@ func (dt DirTree) Add(entry fs.DirEntry) {
 // this creates the directory itself if required
 // it doesn't create parents
 func (dt DirTree) AddDir(entry fs.DirEntry) {
+	dirPath := entry.Remote()
+	if dirPath == "" {
+		return
+	}
 	dt.Add(entry)
 	// create the directory itself if it doesn't exist already
-	dirPath := entry.Remote()
 	if _, ok := dt[dirPath]; !ok {
 		dt[dirPath] = nil
 	}

fs/dirtree/dirtree_test.go

@@ -51,6 +51,14 @@ func TestDirTreeAddDir(t *testing.T) {
 dir/subdir/
   sausage/
 dir/subdir/sausage/
+`, dt.String())
+	d = mockdir.New("")
+	dt.AddDir(d)
+	assert.Equal(t, `/
+  potato/
+dir/subdir/
+  sausage/
+dir/subdir/sausage/
 `, dt.String())
 }