Fix multi selection stage/discard not working for files with substrings (#3599)

- **PR Description**

I found an issue with multi selection stage/discard. Here is a minimal
repro:
```bash
git init
touch a
touch aa
lazygit
```

Select both files using shift and hit space. Only `a` is staged.

- **Please check if the PR fulfills these requirements**

* [ ] Cheatsheets are up-to-date (run `go generate ./...`)
* [x] Code has been formatted (see
[here](https://github.com/jesseduffield/lazygit/blob/master/CONTRIBUTING.md#code-formatting))
* [x] Tests have been added/updated (see
[here](https://github.com/jesseduffield/lazygit/blob/master/pkg/integration/README.md)
for the integration test guide)
* [ ] Text is internationalised (see
[here](https://github.com/jesseduffield/lazygit/blob/master/CONTRIBUTING.md#internationalisation))
* [ ] Docs have been updated if necessary
* [x] You've read through your own file changes for silly mistakes etc

<!--
Be sure to name your PR with an imperative e.g. 'Add worktrees view'
see https://github.com/jesseduffield/lazygit/releases/tag/v0.40.0 for
examples
-->

pkg/gui/controllers/files_controller.go

@@ -1009,10 +1009,14 @@ func normalisedSelectedNodes(selectedNodes []*filetree.FileNode) []*filetree.Fil
 
 func isDescendentOfSelectedNodes(node *filetree.FileNode, selectedNodes []*filetree.FileNode) bool {
 	for _, selectedNode := range selectedNodes {
+		if selectedNode.IsFile() {
+			continue
+		}
+
 		selectedNodePath := selectedNode.GetPath()
 		nodePath := node.GetPath()
 
-		if strings.HasPrefix(nodePath, selectedNodePath) && nodePath != selectedNodePath {
+		if strings.HasPrefix(nodePath, selectedNodePath+"/") {
 			return true
 		}
 	}

pkg/integration/tests/file/stage_children_range_select.go

@@ -0,0 +1,45 @@
+package file
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var StageChildrenRangeSelect = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Stage a range of files/folders and their children using range select",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupConfig: func(config *config.AppConfig) {
+	},
+	SetupRepo: func(shell *Shell) {
+		shell.CreateFile("foo", "")
+		shell.CreateFile("foobar", "")
+		shell.CreateFile("baz/file", "")
+		shell.CreateFile("bazbam/file", "")
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Files().
+			IsFocused().
+			Lines(
+				Contains("▼ baz").IsSelected(),
+				Contains("  ??").Contains("file"),
+				Contains("▼ bazbam"),
+				Contains("  ??").Contains("file"),
+				Contains("??").Contains("foo"),
+				Contains("??").Contains("foobar"),
+			).
+			// Select everything
+			Press(keys.Universal.ToggleRangeSelect).
+			NavigateToLine(Contains("foobar")).
+			// Stage
+			PressPrimaryAction().
+			Lines(
+				Contains("▼ baz").IsSelected(),
+				Contains("  A ").Contains("file").IsSelected(),
+				Contains("▼ bazbam").IsSelected(),
+				Contains("  A ").Contains("file").IsSelected(),
+				Contains("A ").Contains("foo").IsSelected(),
+				Contains("A ").Contains("foobar").IsSelected(),
+			)
+	},
+})

pkg/integration/tests/test_list.go

@@ -160,6 +160,7 @@ var tests = []*components.IntegrationTest{
 	file.DiscardVariousChangesRangeSelect,
 	file.Gitignore,
 	file.RememberCommitMessageAfterFail,
+	file.StageChildrenRangeSelect,
 	file.StageRangeSelect,
 	filter_and_search.FilterCommitFiles,
 	filter_and_search.FilterFiles,