Changed clang-format-diff.py to output diff by default. Added -i option to apply changes to files instead.

Summary:
"svn diff|clang-format-diff.py" will just output the diff.
Now it's possible to use:
  svn diff|clang-format-diff.py|patch -p0
as an equivalent to:
  svn diff|clang-format-diff.py -i
;)

Reviewers: djasper

Reviewed By: djasper

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1840