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

This is a reland of beec66070ea79d5cad14d27927ef1bb2f0f01793

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 <4606aec58cba917397ff2e4738f995e4758694f8@google.com>
> Commit-Queue: Edward Lesmes <d05609f027dc667cc37492b1dc1bdea5d60fa82a@chromium.org>

Bug: 1066187
Change-Id: I8b65549ef47587cef8492f8f76a037eb85a981fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2132972
Reviewed-by: Josip Sokcevic <4606aec58cba917397ff2e4738f995e4758694f8@google.com>
Commit-Queue: Edward Lesmes <d05609f027dc667cc37492b1dc1bdea5d60fa82a@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools