message: Disable context menu 'Merge' and 'Rebase' actions when the repostiory is in a detached HEAD state
  This fixes
diff: @@ -1393,7 +1393,7 @@ void CommitList::contextMenuEvent(QContextMenuEvent *event)
  menu.addSeparator();
  - menu.addAction(tr("Merge..."), [view, commit] {
  + QAction *merge = menu.addAction(tr("Merge..."), [view, commit] {
  MergeDialog *dialog =
  new MergeDialog(RepoView::Merge, view->repo(), view);
  connect(dialog, &QDialog::accepted, [view, dialog] {
  @@ -1408,7 +1408,7 @@ void CommitList::contextMenuEvent(QContextMenuEvent *event)
  dialog->open();
  });
  - menu.addAction(tr("Rebase..."), [view, commit] {
  + QAction *rebase = menu.addAction(tr("Rebase..."), [view, commit] {
  MergeDialog *dialog =
  new MergeDialog(RepoView::Rebase, view->repo(), view);
  connect(dialog, &QDialog::accepted, [view, dialog] {
  @@ -1431,6 +1431,11 @@ void CommitList::contextMenuEvent(QContextMenuEvent *event)
  view->cherryPick(commit);
  });
  + // Disable merge and rebase when HEAD is detached.
  + bool detached = commit.repo().isHeadDetached();
  + merge->setEnabled(!detached);
  + rebase->setEnabled(!detached);
  +
  menu.addSeparator();
  git::Reference head = view->repo().head();
  