You can actually turn on verbose commit messages by using the setting git_commit_verbose. Have a look in the default config for SublimeGit. If you do that, it'll add the diff, and even syntax highlight it for you.
It's not on by default, because it's not the default when using git from the command line. If someone is new to git it might be a little scary to see a wall of text when committing.
I'm not sure that it's actually documented though, which definitely is an error on my part.
