Update shebang for clang-format-diff script to python3.

Different distributions have different strategies migrating the `python` symlink. Debian and its derivatives provide `python-is-python2` and `python-is-python3`. If neither is installed, the user gets no `/usr/bin/python`. The clang-format-diff script and consequently `arc diff` can thus fail with a python not found error.  Since we require python greater than 3.6 as part of llvm prerequisites (https://llvm.org/docs/GettingStarted.html#software), let's go ahead and update this shebang.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D100968