Not sure if via git-webkit script or with a different tool, but if you ammend a local commit, I didn't find a way to regenerate the list of modified files that should be included in the commit message. The only way I found is to "git reset --soft HEAD^" and commit again. But I believe there should be a simpler way to do that.
<rdar://problem/94200273>
I've updated the commit hook now, so if you do `git commit --amend` it should show the full list of modified files in the commented out section of the commit editor.