If checked-out revision is less than 6 hours old, skip fetching latest
revision.

Bug: 1199853
Change-Id: Ifd2a61055de286fe6fd94a5d43f80a1885a6931c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2911015Reviewed-by: Erik Staab <estaab@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>