All of the PRESUBMIT.py files in the Chromium repo are running under
Python 3. However "git cl presubmit" also works with other repos where
some PRESUBMIT.py scripts still run under Python 2. This means that
the Python 2 presubmit commit checks step cannot simply be disabled.
That meant that Chromium was paying up to a one-minute cost just to
setup for and look for Python 2 scripts that it doesn't run.

This change runs the Python 3 PRESUBMIT.py scripts first, and keeps
track of whether any were skipped. If none were skipped then the
Python 2 PRESUBMIT.py stage can be skipped.

Note that the child scripts of PRESUBMIT.py scripts may still be run
under Python 2, but that is orthogonal to this change.

Bug: 1313804
Change-Id: Ib65838223f232f1e78058d6a08ea15a89f442310
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3614453Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>