Add a test for following a file into a rename in a collapsed directory

When the deletion of the selected file is staged and git then reports
it as the old half of a rename whose new half sits in a collapsed
directory, the selection is meant to move to the rename, expanding the
directory on the way. With gui.showRootItemInFileTree turned on, the
directory stays collapsed and the selection lands on it instead. With
the option turned off, the directory does expand, but if another file
in it sorts before the rename, that file gets selected.

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

pkg/gui/filetree/file_tree_view_model_test.go

@@ -78,3 +78,57 @@ func TestSetTreeSelectsNewFileWhenSelectedRenameSplits(t *testing.T) {
 		})
 	}
 }
+
+func TestSetTreeFollowsRenameIntoCollapsedDir(t *testing.T) {
+	scenarios := []struct {
+		name         string
+		showRootItem bool
+		expectedPath string
+	}{
+		{
+			name:         "with root item",
+			showRootItem: true,
+			/* EXPECTED:
+			expectedPath: "a/new.go",
+			ACTUAL: */
+			expectedPath: "a",
+		},
+		{
+			name:         "without root item",
+			showRootItem: false,
+			/* EXPECTED:
+			expectedPath: "a/new.go",
+			ACTUAL: */
+			expectedPath: "a/b.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: "a/b.go"},
+				{Path: "a/new.go"},
+				{Path: "old.go"},
+			}
+			viewModel := NewFileTreeViewModel(func() []*models.File { return files }, cmn, true)
+			viewModel.SetTree()
+			viewModel.ToggleCollapsed(InternalTreePathForFilePath("a", s.showRootItem))
+			idx, found := viewModel.GetIndexForPath(InternalTreePathForFilePath("old.go", s.showRootItem))
+			assert.True(t, found)
+			viewModel.SetSelection(idx)
+
+			// staging the deletion of old.go turns it into the old half of a rename
+			files = []*models.File{
+				{Path: "a/b.go"},
+				{Path: "a/new.go", PreviousPath: "old.go"},
+			}
+			viewModel.SetTree()
+
+			assert.Equal(t, s.expectedPath, viewModel.GetSelectedPath())
+		})
+	}
+}