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/+/2829942Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>