Check that the cache contains the needed refs, and not only the revisions,
before attempting to checkout.

Bug: 910547
Change-Id: Ia269132421994803866065e81fc21ecd7ac206ed
Reviewed-on: https://chromium-review.googlesource.com/c/1362393Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>