X-debbugs-cc: dgutov@yandex.ru, juri@linkov.net

These patches add a new command, diff-discard-hunk, inspired by some
functionality from Magit.  It nicely complements a workflow based around
committing with C-x v v from C-x v D.  I have been using it every day
for a year or so, and I think others will find it useful, too.

Posting for comments.  Thanks!

-- 
Sean Whitton