git-cl: Execute clang-format-diff.py using vpython.

When running git-cl using vpython3, sys.executable is python3.
Execute clang-format-diff.py using vpython instead, since it has
not been migrated already.

Bug: 1066187
Change-Id: I31556acb50ff4256706a313c1393eddad9f903f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2130926
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

git_cl.py

@@ -4758,7 +4758,7 @@ def _RunClangFormatDiff(opts, clang_diff_files, top_dir, upstream_commit):
     except clang_format.NotFoundError as e:
       DieWithError(e)
 
-    cmd = [sys.executable, script, '-p0']
+    cmd = ['vpython', script, '-p0']
     if not opts.dry_run and not opts.diff:
       cmd.append('-i')