[git-clang-format] Change run line from python to python3

[thakis]

Summary

Several systems no longer ship python.

[curdeius]

LGTM. Especially that we already use python3 in clang-format-diff and other scripts, and it's used in many places in LLVM.

clang/tools/clang-format/git-clang-format

-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 #===- git-clang-format - ClangFormat Git Integration ---------*- python -*--===#
 #
 # Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 # See https://llvm.org/LICENSE.txt for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 #
 #===------------------------------------------------------------------------===#