This avoids diffs being applied in the work tree to files that are
supposed to be excluded (clang tests), allows arc to properly provide
interactive feedback for the formatting fixes, and reduces the number of
files that we format, in a change affecting N files, from N^2 to N.

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

This reverts commit 31b6e182f2ce957c86b961e21b9eb82bbcea7c59.

As it was breaking `arc diff` workflow for multiple people.

LLVM includes .clang-format files which must be respected.

D49116 was using clang-format-diff because at the time of its writing,
it needed to handle the subversion repo as well.

Differential Revision: https://reviews.llvm.org/D77363

Summary:
This uses clang-format-diff as a linter for Arcanist.

`arc lint` flow, also run as part of `arc diff` unless skipped with
`--nolint`, will now run the linter shell script on the changed files,
and prompt the user to accept the suggested changes.

Message when clang-format-diff is not installed:
{F6654094}

Example of the noise during code review when clang-format-diff is not installed:
https://reviews.llvm.org/differential/changeset/?ref=1115809

Prompt when clang-format-diff is installed and suggests edits:
{F6650223}

Reviewers: probinson, scott.linder

Reviewed By: scott.linder

Subscribers: scott.linder, MyDeveloperDay, JonasToth, danilaml, JDevlieghere, dberris, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D49116