Add a test demonstrating that you can't unstage a dirty submodule

When a submodule has both a new commit (which the parent repo can stage)
and dirty working-tree content (which it can't), staging it lands on a
"MM" status. Pressing space again should unstage it, but instead it tries
to stage the dirty content over and over, so you can never get back to an
unstaged state.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

pkg/integration/tests/submodule/stage.go

@@ -0,0 +1,51 @@
+package submodule
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var Stage = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Stage and unstage a submodule that has both a new commit and dirty content. The new commit can be staged, but the dirty content can't, so unstaging must still work.",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupConfig: func(config *config.AppConfig) {
+		config.GetUserConfig().Gui.ShowFileTree = false
+	},
+	SetupRepo: func(shell *Shell) {
+		shell.EmptyCommit("first commit")
+		shell.CloneIntoSubmodule("my_submodule_name", "my_submodule_path")
+		shell.GitAddAll()
+		shell.Commit("add submodule")
+
+		// Give the submodule a new commit, which is a change that the parent
+		// repo can stage, as well as some dirty working-tree content, which
+		// the parent repo can never stage. This is what gets us a "MM" status
+		// once the new commit is staged.
+		shell.RunCommand([]string{"git", "-C", "my_submodule_path", "commit", "--allow-empty", "-m", "submodule commit"})
+		shell.CreateFile("my_submodule_path/dirty_file", "dirty content")
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Files().Focus().
+			Lines(
+				Equals(" M my_submodule_path (submodule)").IsSelected(),
+			).
+			// Staging the submodule stages the new commit, but the dirty
+			// content remains unstaged, leaving us at "MM".
+			PressPrimaryAction().
+			Lines(
+				Equals("MM my_submodule_path (submodule)").IsSelected(),
+			).
+			// Pressing again must unstage the submodule, taking us back to
+			// " M" rather than trying (and failing) to stage the dirty content.
+			PressPrimaryAction().
+			/* EXPECTED:
+			Lines(
+				Equals(" M my_submodule_path (submodule)").IsSelected(),
+			)
+			ACTUAL: */
+			Lines(
+				Equals("MM my_submodule_path (submodule)").IsSelected(),
+			)
+	},
+})

pkg/integration/tests/test_list.go

@@ -426,6 +426,7 @@ var tests = []*components.IntegrationTest{
 	submodule.RemoveNested,
 	submodule.Reset,
 	submodule.ResetFolder,
+	submodule.Stage,
 	sync.FetchAndAutoForwardBranchesAllBranches,
 	sync.FetchAndAutoForwardBranchesAllBranchesCheckedOutInOtherWorktree,
 	sync.FetchAndAutoForwardBranchesNone,