This is a reland of d3affaa6

Original change's description:
> Use OS level locking in git_cache.py
> 
> Without OS level locking it's possible to leave "lock" files on disk
> which will prevent next run to acquire those locks. This can easily
> happen if SIGKIL is issued.
> 
> R=apolito@google.com, ehmaldonado@chromium.org
> 
> Bug: 1049610
> Change-Id: Id87aa1376b9ea5ff0c2d14f3603636493ed1dd5b
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2189333
> Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
> Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
> Commit-Queue: Josip Sokcevic <sokcevic@google.com>

Bug: 1049610
Change-Id: I58e65a10f7c779e0de1121ba7167c694996e390c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2211189Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>