"git commit -a" is what I call a cowboy-commit, and I still think git should flag and expose commits as such.
Not that I mind anyone using it (carefully), but it would make me extra wary when reviewing a commit. It should be the absolute exception, not the standard way of committing things.
Do it properly and always review your individual changes once more! Use "git add -p"!
You will soon love it and whoever reviews your code will love you for using it!