RFE: Discard hunks during `git add -p`

Current version: 2.10.2 Example workflow:

* I would do a global substitution across a source tree, e.g. `perl -i 
  -pe 's{OLD_FOO\(x\)}{NEW_BAR(x, 0)}' *.c`
* Using `git add -p`, I would verify each of the substitutions that they 
  make sense in their respective locations, and, based on that,
  answer "y" or "n" to the interactive prompting to stage good hunks.
* When done with add-p, I would commit the so-staged hunks,
  and then use `git reset --hard` to discard all changes that were 
  not acknowledged during add-p.

Being able to discard hunks (reset working copy to index contents) during add-p would alleviate the (quite broad) hard reset.

Similar approach:

* global substitution
* Using `git add -p`, some hunks may warrant some more editing, doable 
  with the "e" command. The index would be updated with the extra
  change, but the working copy be left as-is.
* When rerunning `git add -p` in such a state, a difference is shown 
  again for such edited spots, which I would like to discard (bring 
  the working copy into sync with index).