Beej's Guide to Git

I usually use :q! which seems to do the same thing

The minor difference is that :q! quits without saving but returns zero as the exit code, but :cq quits with a nonzero exit code. Git interprets the nonzero exit code as "editing failed", following the Unix convention that zero means success. If you didn't save the commit message while working on it, :q! will send the empty template back to Git, which Git is smart enough to not commit. But if you accidentally save your work partway through, :q! will still commit the message you wanted to abandon.

That only works if the edit buffer is blank or only has commented out lines. In other cases, such as when you're trying to cancel a `git commit --amend` that loads up the last commit message in your buffer, :q! will not cancel the commit, but :cq will.