Add a test for the selection after a selected rename splits in two

When a rename is selected in the files panel and then splits into its
two halves, e.g. because it was unstaged, the selection is meant to move
to the new half. This only works with gui.showRootItemInFileTree turned
off. With the default setting, the selection lands on the file that
follows the rename in the list instead.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

pkg/gui/filetree/file_tree_view_model_test.go

@@ -5,6 +5,7 @@ import (
 
 	"github.com/jesseduffield/lazygit/pkg/commands/models"
 	"github.com/jesseduffield/lazygit/pkg/common"
+	"github.com/jesseduffield/lazygit/pkg/config"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -30,3 +31,53 @@ func TestSetStatusFilterPreservingSelection(t *testing.T) {
 	assert.Equal(t, "file3", viewModel.GetSelectedPath())
 	assert.False(t, viewModel.IsSelectingRange())
 }
+
+func TestSetTreeSelectsNewFileWhenSelectedRenameSplits(t *testing.T) {
+	scenarios := []struct {
+		name         string
+		showRootItem bool
+		expectedPath string
+	}{
+		{
+			name:         "with root item",
+			showRootItem: true,
+			/* EXPECTED:
+			expectedPath: "dir/new.go",
+			ACTUAL: */
+			expectedPath: "other.go",
+		},
+		{
+			name:         "without root item",
+			showRootItem: false,
+			expectedPath: "dir/new.go",
+		},
+	}
+
+	for _, s := range scenarios {
+		t.Run(s.name, func(t *testing.T) {
+			userConfig := config.GetDefaultConfig()
+			userConfig.Gui.ShowRootItemInFileTree = s.showRootItem
+			cmn := common.NewDummyCommonWithUserConfigAndAppState(userConfig, nil)
+
+			files := []*models.File{
+				{Path: "dir/new.go", PreviousPath: "dir/old.go"},
+				{Path: "other.go"},
+			}
+			viewModel := NewFileTreeViewModel(func() []*models.File { return files }, cmn, true)
+			viewModel.SetTree()
+			idx, found := viewModel.GetIndexForPath(InternalTreePathForFilePath("dir/new.go", s.showRootItem))
+			assert.True(t, found)
+			viewModel.SetSelection(idx)
+
+			// the rename is split into its two halves, e.g. because it was unstaged
+			files = []*models.File{
+				{Path: "dir/new.go"},
+				{Path: "dir/old.go"},
+				{Path: "other.go"},
+			}
+			viewModel.SetTree()
+
+			assert.Equal(t, s.expectedPath, viewModel.GetSelectedPath())
+		})
+	}
+}