Better error message when trying to edit or move a non-todo commit during rebase

Previously we would have tried to do the rebase, resulting in a long and
somewhat cryptic error message from git; now we check ourselves and show a less
intimidating message.

pkg/integration/tests/interactive_rebase/edit_non_todo_commit_during_rebase.go

@@ -0,0 +1,37 @@
+package interactive_rebase
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var EditNonTodoCommitDuringRebase = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Tries to edit a non-todo commit while already rebasing, resulting in an error message",
+	ExtraCmdArgs: "",
+	Skip:         false,
+	SetupConfig:  func(config *config.AppConfig) {},
+	SetupRepo: func(shell *Shell) {
+		shell.
+			CreateNCommits(2)
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Commits().
+			Focus().
+			Lines(
+				Contains("commit 02").IsSelected(),
+				Contains("commit 01"),
+			).
+			Press(keys.Universal.Edit).
+			Lines(
+				Contains("<-- YOU ARE HERE --- commit 02"),
+				Contains("commit 01"),
+			).
+			NavigateToLine(Contains("commit 01")).
+			Press(keys.Universal.Edit)
+
+		t.ExpectPopup().Alert().
+			Title(Equals("Error")).
+			Content(Contains("Can't perform this action during a rebase")).
+			Confirm()
+	},
+})