Update git checkout before updating the tools.
If the scripts to update the tools are broken in the current revision,
this will allow us to get a new, hopefully working, revision instead of
leaving us stuck on a broken one.

We already do this for Linux.

Change-Id: I31856ebbd659d5360ad705dc39e5856b4140a19f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2208625Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>