Tweak selection-undo interaction with WinDisplay hooks

Each selection undo operation is surrounded by pair of
begin_edition()/end_edition() calls.
The original reason for adding these was that in one of my preliminary
versions, a WinDisplay hook could break an undo chain, even if the
hook did not affect selections at all. This has since been fixed.

By surrounding the undo with begin_edition()/end_edition(), try to
ensure that any selection modification that happens in a WinDisplay
hook would not break the undo chain. Essentially this means that,
after using <c-h> to undo a buffer change, this was meant to
make sure that <c-k> could redo that buffer change.

However, it turns out this actually doesn't work.  The attached test
case triggers an assertion.  As described in the first paragraph,
the only real-world motivation for this is gone, so let's simplify
the behavior.
The assertion fix means that we can test the next commit better.

src/context.cc

@@ -225,11 +225,6 @@ void Context::SelectionHistory::undo()
     if (in_edition())
         throw runtime_error("selection undo is only supported at top-level");
     kak_assert(not empty());
-    begin_edition();
-    auto end = on_scope_end([&] {
-        kak_assert(current_history_node().selections == m_staging->selections);
-        end_edition();
-    });
     HistoryId next;
     if constexpr (backward)
         next = current_history_node().parent;
@@ -242,7 +237,6 @@ void Context::SelectionHistory::undo()
         m_history_id = next;
         if constexpr (backward)
             current_history_node().redo_child = previous_id;
-        m_staging = current_history_node();
     };
     Buffer& destination_buffer = history_node(next).selections.buffer();
     if (&destination_buffer == &m_context.buffer())

test/normal/selection-undo/windisplay-hook/cmd

@@ -0,0 +1 @@
+

test/normal/selection-undo/windisplay-hook/in

@@ -0,0 +1,3 @@
+1
+2
+3

test/normal/selection-undo/windisplay-hook/out

@@ -0,0 +1,3 @@
+1
+2
+here3

test/normal/selection-undo/windisplay-hook/rc

@@ -0,0 +1 @@
+hook global WinDisplay .*/out %{exec j}

test/normal/selection-undo/windisplay-hook/script

@@ -0,0 +1,2 @@
+ui_out -ignore 4
+ui_in '{ "jsonrpc": "2.0", "method": "keys", "params": [ "j:buffer *debug*<ret><c-h>ihere<esc>" ] }'