Earlier quoted context omitted.
It's simply not necessary to have that feature. Sapling encourages regularly committing and amending commits rather than staging changes. It's also easy to commit / amend part of your work by selecting the lines to include in nice curses interface (--interactive).
I agree it's not necessary, but i like having it because it lets me separate what's going to be added before i actually commit. I still commit small, frequent. But i like `git add -p` to skip debug lines, hardcoded conditions, etc. I don't want to mistakenly auto commit a whole pile of lines and then have to remove debugs/hacks/etc from things i've committed. Stage + Unstaged is my working area, and the two live toge…
You can just use the tip as your staging. Use interactive amending to move changes from the working copy to the commit, and when you want to "commit", finish up the message.
hg actually has an "unamend" command (part of the "uncommit" standard extension) which... reverts the last amend. Rather than having to remember how to contort reset into the right shape to move changes back out of staging without destroying everything.