Here are the commit messages written by the developers of Git itself: https://git.kernel.org/pub/scm/git/git.git/log/

The most recent commits are a load of merges, but the slightly older ones have very long commit messages. The first one I clicked [1] has 11 changed lines of code, and a 32-line commit message.

[1] https://git.kernel.org/pub/scm/git/git.git/commit/?id=23c431...