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

Change-Id: I71689e30bdede392588c69e118e9297d86a134a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2120281Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>