If you've a big dirty git workspace, with loads of documents with whitespace (which you know you can safely commit), you can use python's sh library (https://github.com/amoffat/sh) and the script below to safely stage all files with only whitespace changes

git_whitespace.py

#!/usr/bin/python
import sh

git = sh.git.bake('--no-pager',  # @UndefinedVariable
                  _cwd='/Developer/repos/git/example')

for fname in git.diff('--name-only', '--full-index', 'HEAD').splitlines():
    if '' == git.diff('-w', fname):
        print 'adding', fname
        git.add(fname)