This is a reland of 4c67f856
Issues should have been fixed by crrev.com/c/2838026

Original change's description:
> git-cache: Add option to fetch commits.
>
> Add option to git cache to fetch commits.
> And use it in bot_update and gclient sync to make sure
> the needed commits are present on the checkout.
>
> Change-Id: I9e90da9e3be6e7bacf714b22bf0b735463e655b6
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2829942
> Reviewed-by: Gavin Mak <gavinmak@google.com>
> Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

Change-Id: Ie5a29737f5a75d28bc7c5c2f6cb99ec7f87cd9e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2841046Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>