Git is automatically paging the output for you, since logs tend to easily overflow a single terminal window size (you’re in one of the rare exceptions – a oneline format and a small commit limit). If you don’t want this, use:
git --no-pager log -n 20 --pretty=oneline
Note that this does mean you’ll get some ugly wrapping, because the pager was previously turning off wrapping for you (since you could use the cursor keys to scroll left-right).