I don't use hg much these days but you can just pass the file(s) you want to commit to 'hg commit'; no need for staging. Even better, hg (or its shell integration at any rate) has tab completion so it's easy to go though the modified files and pick those you wanted to commit. I kinda miss this now I'm working with git mostly.
I can't tell you how many times I've shot myself in the foot accidentally committing stuff to svn when I just meant a single file.