Add a test that demonstrates the problem

Using the "Add to .git/info/exclude" in a worktree results in an error message,
as the test shows. The same would happen in a submodule, but I'm not adding an
extra test for that, as the circumstances are the same.

pkg/integration/tests/test_list.go

@@ -308,6 +308,7 @@ var tests = []*components.IntegrationTest{
 	worktree.DetachWorktreeFromBranch,
 	worktree.DotfileBareRepo,
 	worktree.DoubleNestedLinkedSubmodule,
+	worktree.ExcludeFileInWorktree,
 	worktree.FastForwardWorktreeBranch,
 	worktree.ForceRemoveWorktree,
 	worktree.RemoveWorktreeFromBranch,

pkg/integration/tests/worktree/exclude_file_in_worktree.go

@@ -0,0 +1,46 @@
+package worktree
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var ExcludeFileInWorktree = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Add a file to .git/info/exclude in a worktree",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupConfig:  func(config *config.AppConfig) {},
+	SetupRepo: func(shell *Shell) {
+		shell.EmptyCommit("commit1")
+		shell.AddWorktree("HEAD", "../linked-worktree", "mybranch")
+		shell.CreateFile("../linked-worktree/toExclude", "")
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Worktrees().
+			Focus().
+			Lines(
+				Contains("repo (main)").IsSelected(),
+				Contains("linked-worktree"),
+			).
+			SelectNextItem().
+			PressPrimaryAction()
+
+		t.Views().Files().
+			Focus().
+			Lines(
+				Contains("toExclude"),
+			).
+			Press(keys.Files.IgnoreFile).
+			Tap(func() {
+				t.ExpectPopup().Menu().Title(Equals("Ignore or exclude file")).Select(Contains("Add to .git/info/exclude")).Confirm()
+			}).
+			/* EXPECTED:
+			IsEmpty()
+
+			t.FileSystem().FileContent("../repo/.git/info/exclude", Contains("toExclude"))
+			ACTUAL: */
+			Tap(func() {
+				t.ExpectPopup().Alert().Title(Equals("Error")).Content(Contains("open .git/info/exclude: not a directory"))
+			})
+	},
+})