refactor(aucmd_win): remove need to restore window layout

There are some places that mess with the window layout in preparation for moving
a window to a different split (win_split_ins called with new_wp != NULL).

This means the window layout can change slightly even if win_split_ins fails.

This is why it was still needed to restore the window layout in
aucmd_{prep,rest}buf even if we disallow win_split_ins from making aucmd_win
non-floating by moving it into a split.

We can just skip messing with the layout in such places if we're dealing with
the aucmd_win.

src/nvim/autocmd.c

@@ -1147,7 +1147,6 @@ void aucmd_prepbuf(aco_save_T *aco, buf_T *buf)
     globaldir = NULL;
 
     block_autocmds();  // We don't want BufEnter/WinEnter autocommands.
-    make_snapshot(SNAP_AUCMD_IDX);
     if (need_append) {
       win_append(lastwin, aucmd_win);
       pmap_put(handle_T)(&window_handles, aucmd_win->handle, aucmd_win);
@@ -1213,8 +1212,6 @@ win_found:
       close_tabpage(curtab);
     }
 
-    restore_snapshot(SNAP_AUCMD_IDX, false);
-    win_comp_pos();  // recompute window positions
     unblock_autocmds();
 
     win_T *const save_curwin = win_find_by_handle(aco->save_curwin_handle);

src/nvim/eval/funcs.c

@@ -4242,7 +4242,7 @@ static void win_move_into_split(win_T *wp, win_T *targetwin, int size, int flags
   int height = wp->w_height;
   win_T *oldwin = curwin;
 
-  if (wp == targetwin) {
+  if (wp == targetwin || wp == aucmd_win) {
     return;
   }
 

src/nvim/window.c

@@ -1838,6 +1838,9 @@ static void win_totop(int size, int flags)
     beep_flush();
     return;
   }
+  if (curwin == aucmd_win) {
+    return;
+  }
 
   if (curwin->w_floating) {
     ui_comp_remove_grid(&curwin->w_grid_alloc);