Makes it possible to use vimdiff (or Meld, or whatever) instead of git add -p/git add -e for fine-grained staging.
If file is not staged yet, opens regular $EDITOR.
# defaults to vimdiff
git config --add --global vimstage.editor meld
# or, if you want to try another editor
export DEDITOR=meld
git vimstage ./foo
# and see what you did:
git diff --cached ./foo