pagers to the rescue
git log | less
Make sure you don’t have -S on an alias for less
Also, it is generally considered good practice to limit the width for commit messages. I believe a common standard is 78 chars (IIRC), and most texteditors do a good job of ensuring such style rules (auto formatting your message).
Update: As a reference data point, git-config lists:
gui.commitmsgwidth
Defines how wide the commit message window is in the git-gui(1). "75" is the default.