This can happen if the cache repo was init'd, but has no pack files;
previously it would try to fetch from scratch.

R=ehmaldonado@chromium.org, tandrii@chromium.org

No-Presubmit: true
Change-Id: I97e863dd6c2ecfd00fdb4238dda48e0d3929c4f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1941337
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@google.com>