ex_getln.c: fix compute_cmdrow() not resetting lines_left (#10749)

Before this commit, when `inccomand` was set to `nosplit`, multi-line
substitutions collapsed the command-line.

This happened because when ex_getln.c:cursorcmd() computed a msg_row, it
was given a cmdline_row one line too high. This happened because
message.c:msg_puts_display() was supposed to decrement cmdline_row but
didn't, because of the `msg_no_more && lines_left == 0` check placed
just before the decrementation part in msg_puts_display's while loop.

Every time msg_puts_display writes a line, it decreases `lines_left` (a
variable used to know how many lines are left for prompts). Since
redrawcommandline() did not reset `lines_left` between calls to
msg_puts_display, every time a character was pressed, `lines_left` was
decremented. This meant that once the user pressed COLUMNS+ROWS numbers
of characters, `lines_left` would reach 0 and prevent msg_row from being
decremented.

It makes sense to fix setting `lines_left` to `cmdline_row` in
`compute_cmdrow` ; after all, computing where the command line row
should be placed is equivalent to computing how many `lines_left` of
output there are left.

Closes #8254.

src/nvim/ex_getln.c

@@ -3483,6 +3483,7 @@ void compute_cmdrow(void)
     cmdline_row = wp->w_winrow + wp->w_height
                   + wp->w_status_height;
   }
+  lines_left = cmdline_row;
 }
 
 static void cursorcmd(void)