Set groundwork for better disabled reasons with range select

Something dumb that we're currently doing is expecting list items
to define an ID method which returns a string. We use that when copying
items to clipboard with ctrl+o and when getting a ref name for diffing.

This commit gets us a little deeper into that hole by explicitly requiring
list items to implement that method so that we can easily use the new
helper functions in list_controller_trait.go.

In future we need to just remove the whole ID thing entirely but I'm too
lazy to do that right now.

pkg/gui/filetree/file_tree_view_model.go

@@ -7,6 +7,7 @@ import (
 	"github.com/jesseduffield/lazygit/pkg/gui/context/traits"
 	"github.com/jesseduffield/lazygit/pkg/gui/types"
 	"github.com/jesseduffield/lazygit/pkg/utils"
+	"github.com/samber/lo"
 	"github.com/sirupsen/logrus"
 )
 
@@ -43,6 +44,36 @@ func (self *FileTreeViewModel) GetSelected() *FileNode {
 	return self.Get(self.GetSelectedLineIdx())
 }
 
+func (self *FileTreeViewModel) GetSelectedItemId() string {
+	item := self.GetSelected()
+	if item == nil {
+		return ""
+	}
+
+	return item.ID()
+}
+
+func (self *FileTreeViewModel) GetSelectedItems() ([]*FileNode, int, int) {
+	startIdx, endIdx := self.GetSelectionRange()
+
+	nodes := []*FileNode{}
+	for i := startIdx; i <= endIdx; i++ {
+		nodes = append(nodes, self.Get(i))
+	}
+
+	return nodes, startIdx, endIdx
+}
+
+func (self *FileTreeViewModel) GetSelectedItemIds() ([]string, int, int) {
+	selectedItems, startIdx, endIdx := self.GetSelectedItems()
+
+	ids := lo.Map(selectedItems, func(item *FileNode, _ int) string {
+		return item.ID()
+	})
+
+	return ids, startIdx, endIdx
+}
+
 func (self *FileTreeViewModel) GetSelectedFile() *models.File {
 	node := self.GetSelected()
 	if node == nil {