… <CR> (#36334)

Problem:  Cmdline history not updated when mapping both <Up> and <CR>.
Solution: Consider the command typed when in Cmdline mode and there is
          no pending input (zeertzjq).

Although the existing behavior technically does match documentation, the
"completely come from mappings" part is a bit ambiguous, because one may
argue that the command doesn't completely come from mappings as long as
the user has typed a key in Cmdline mode.  I'm not entirely sure if this
change will cause problems, but it seems unlikely.

fixes: vim/vim#2771
related: #36256
closes: vim/vim#18607

vim/vim@97b6e8b
(cherry picked from commit 2407833)