Use in-tree clang-format-diff.py as Arcanist linter

Summary:
The only guarantee there seems to be in the clang-format packaging is
that an executable called `clang-format` is in the PATH. Use the
in-tree `clang-format-diff.py` to avoid assuming anything else.

Also remove dead code for SVN repo and switch to `git diff-index` which
is the git plumbing equivalent of `git diff` in this case.

Reviewers: starsid, mehdi_amini, vitalybuka, fhahn, kadircet

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77428