This new logic tries to figure out which files really changed in a DEPS roll by checking out the old DEPS file and recursively running git diff on all of the repos listed in DEPS. When successful, affected_files turns from just ['DEPS'] into an actual list of affected files. When not successful, it falls back to the original logic.

When DEPS is autorolled, typically the change is just an updated git revision. We don't apply this logic to manual DEPS rolls, as they can contain changes to other variables, whose effects are not easily discernible.

MUST be applied in tandem with https://crrev.com/c/2250928

Bug: 923016
Change-Id: I9e33461ea4ae81dc056d229f8604943745e30389
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2283855
Commit-Queue: Gregory Guterman <guterman@google.com>
Reviewed-by: Garrett Beaty <gbeaty@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>