edit.c: Ensure undo sync when emulating <Esc>x #11706

After PR #8226 an unmapped META key in insert mode behaves like
ESC-<key> (:help i_META).

The behaviour does not fully match, since if <Esc>-<key> is pressed
manually then since it were pressed manually `gotchars` would be called
on the second <key> after insert-mode had already been left.

This would mean that `may_sync_undo` (called from `gotchars`) would
call `u_sync(FALSE)` on the second key (since we would be in normal
mode).

This overall means that <Meta-[something]> behaves differently with
respect to undo than <Esc>[something] when the [something] makes a
change.

As an example, under `nvim -u NONE`:
ihello<M-.>u

leaves the buffer empty, while
ihello<Esc>.u

leaves the buffer with one instance of `hello`.

- Fix by calling u_sync() manually in the new clause under
  `normalchar:` in `insert_handle_key`.
- Update test in tui_spec.lua that accidentally relied on the old behaviour.

src/nvim/edit.c

@@ -1234,6 +1234,7 @@ static int insert_handle_key(InsertState *s)
       // Unmapped ALT/META chord behaves like ESC+c. #8213
       stuffcharReadbuff(ESC);
       stuffcharReadbuff(s->c);
+      u_sync(false);
       break;
     }
 

test/functional/insert/insert_spec.lua

@@ -37,5 +37,11 @@ describe('insert-mode', function()
     command('iunmap <M-l>')
     feed('0i<M-l>')
     eq({ 0, 1, 2, 0, }, funcs.getpos('.'))
+    -- Unmapped ALT-chord has same `undo` characteristics as ESC+<key>
+    command('0,$d')
+    feed('ahello<M-.>')
+    expect('hellohello')
+    feed('u')
+    expect('hello')
   end)
 end)

test/functional/terminal/tui_spec.lua

@@ -299,6 +299,8 @@ describe('TUI', function()
     feed_data('u')
     expect_child_buf_lines({'"pasted from terminal"'})
     feed_data('u')
+    expect_child_buf_lines({'""'})
+    feed_data('u')
     expect_child_buf_lines({''})
   end)