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.
