Add test demonstrating the problem

When dropping changes to the submodule, we expect it to get rolled back to the
previous version; however, it is removed entirely instead.

pkg/integration/tests/commit/discard_submodule_changes.go

@@ -0,0 +1,57 @@
+package commit
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var DiscardSubmoduleChanges = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Discarding changes to a submodule from an old commit.",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupConfig:  func(config *config.AppConfig) {},
+	SetupRepo: func(shell *Shell) {
+		shell.EmptyCommit("Initial commit")
+		shell.CloneIntoSubmodule("submodule", "submodule")
+		shell.Commit("Add submodule")
+
+		shell.AddFileInWorktreeOrSubmodule("submodule", "file", "content")
+		shell.CommitInWorktreeOrSubmodule("submodule", "add file in submodule")
+		shell.GitAdd("submodule")
+		shell.Commit("Update submodule")
+
+		shell.UpdateFileInWorktreeOrSubmodule("submodule", "file", "changed content")
+		shell.CommitInWorktreeOrSubmodule("submodule", "change file in submodule")
+		shell.GitAdd("submodule")
+		shell.Commit("Update submodule again")
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Commits().
+			Focus().
+			Lines(
+				Contains("Update submodule again").IsSelected(),
+				Contains("Update submodule"),
+				Contains("Add submodule"),
+				Contains("Initial commit"),
+			).
+			PressEnter()
+
+		t.Views().CommitFiles().
+			IsFocused().
+			Lines(
+				Equals("M submodule").IsSelected(),
+			).
+			Press(keys.Universal.Remove)
+
+		t.ExpectPopup().Confirmation().
+			Title(Equals("Discard file changes")).
+			Content(Contains("Are you sure you want to remove changes to the selected file(s) from this commit?")).
+			Confirm()
+
+		t.Shell().RunCommand([]string{"git", "submodule", "update"})
+		/* EXPECTED:
+		t.FileSystem().FileContent("submodule/file", Equals("content"))
+		ACTUAL: */
+		t.FileSystem().PathNotPresent("submodule/file")
+	},
+})