Check out this mercurial tip. To cite the link - if you have accidentally added a file, the way to undo that (changing its status from A back to ?, or unknown) is hg revert. For example, if you just ran hg add and realized that you do not want files foo or bar to be tracked by Mercurial:
hg revert foo bar
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…