Change git cache to stop zipping the git directory and instead upload directly to GS.

Bug: 943696
Change-Id: I310fb48ff3d7fd6781914e14e0e17530d5d9b328
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1585029
Commit-Queue: Karen Qian <karenqian@google.com>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>