This will let developers run PRESUBMIT against all files in the
repo, rather than just those their CL touches. This will be
particularly helpful when modifying PRESUBMIT.py itself.

R=maruel@chromium.org

Bug: 519046
Change-Id: I5775eef8aecefe0cd358537c9218afc7480a7619
Reviewed-on: https://chromium-review.googlesource.com/798056
Commit-Queue: Aaron Gable <agable@chromium.org>
Reviewed-by: Marc-Antoine Ruel <maruel@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>