patch 9.1.1969: Wrong cursor position after formatting with long 'formatprg'

patch 9.1.1969: Wrong cursor position after formatting with long 'formatprg'

Problem:  Wrong cursor position after formatting with long 'formatprg'.
Solution: Don't show hit-enter prompt when there are stuffed characters.

Previously a stuffed character at the hit-enter prompt will dismiss the
prompt immediately and be put in the typeahead buffer, which leads to
incorrect behavior as the typeahead buffer is processed after the stuff
buffers. Using vungetc() when KeyStuffed is TRUE can fix this problem,
but since the hit-enter prompt isn't visible anyway (and is likely not
desired here), just skip the prompt instead, which also avoids a wait
when using "wait" instead of "hit-enter" in 'messagesopt'.

fixes:  #18905
closes: #18906

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>