Currently on the wiki (on https://trac.webkit.org/wiki/UsingGitWithWebKit) we recommend setting the core.editor config variable to generate commit messages.

It would seem better to not override the editor configuration, but to instead use the hook that git provides for such tasks (prepare-commit-msg). We will need some level of complexity in the script to handle the different sources of commit message (i.e., the second argument to the hook).