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

This is a reland of beec6607

Append the clang-format directory to the front of PATH instead of
overwriting it.
This allows vpython to be invoked.

Original change's description:
> 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>

Bug: 1066187
Change-Id: I8b65549ef47587cef8492f8f76a037eb85a981fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2132972Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

git_cl.py

@@ -4750,21 +4750,22 @@ def _RunClangFormatDiff(opts, clang_diff_files, top_dir, upstream_commit):
       if opts.diff:
         sys.stdout.write(stdout)
   else:
-    env = os.environ.copy()
-    env['PATH'] = str(os.path.dirname(clang_format_tool))
     try:
       script = clang_format.FindClangFormatScriptInChromiumTree(
           'clang-format-diff.py')
     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')
 
     diff_cmd = BuildGitDiffCmd('-U0', upstream_commit, clang_diff_files)
     diff_output = RunGit(diff_cmd).encode('utf-8')
 
+    env = os.environ.copy()
+    env['PATH'] = (
+        str(os.path.dirname(clang_format_tool)) + os.pathsep + env['PATH'])
     stdout = RunCommand(cmd, stdin=diff_output, cwd=top_dir, env=env)
     if opts.diff:
       sys.stdout.write(stdout)