shallow mode is a special git mode that only checks out a shallow copy
of git so the full history is not downloaded. It is not well supported
by googlesource.com servers because git has to do a giant negotiation
with the server and transmit every single commit that the local checkout
has or doesn't have (instead of just the latest commit of each ref),
which ends up taking more than 10x longer than a non-shallow checkout.

R=ehmaldonado, hinoka, iannucci

Recipe-Nontrivial-Roll: infra
Recipe-Nontrivial-Roll: build
Recipe-Nontrivial-Roll: build_limited_scripts_slave
Bug: 855137
Change-Id: I5f9e31f8b2730cf80b79bfd0e08201c33eb942f5
Reviewed-on: https://chromium-review.googlesource.com/1110450
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>