Refactor gitg base code for multiple selection
After introducing multiple selection, the gitg code base must be refactored, since when we're retrieving a commit we use the selection.get_selected()
method e.g. https://gitlab.gnome.org/mahmoudkhalil11/gitg/-/blob/master/gitg/history/gitg-history.vala#L714 and https://gitlab.gnome.org/mahmoudkhalil11/gitg/-/blob/master/gitg/commit/gitg-commit.vala#L1894
The get_selected()
method won't operate correctly when the selection mode is set to MULTIPLE, so we need to figure a way to retrieve the right commit in cases like these