patch 9.2.0806: 'showcmd' may show internal command keys

patch 9.2.0806: 'showcmd' may show internal command keys

Problem:  The `showcmd` statusline item may show internal command keys when a
          `<Cmd>` or `<ScriptCmd>` mapping redraws the statusline, and may
          leave stale text behind when `%S` is rendered directly.
Solution: Do not add these internal mapping dispatch keys to the `showcmd`
          buffer, and keep the clear state in sync when `%S` renders it
          (Barrett Ruth)

closes: #20769

Co-authored-by: zeertzjq <35768171+zeertzjq@users.noreply.github.com>
Signed-off-by: Barrett Ruth <br@barrettruth.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>