Includes:
* fixes to `library.sh` in order to have some `hack` scripts working again;
* better checking of PRs where presubmit tests are unnecessary;
Bonus: adds the missing eXecution bit to `update-deps.sh`.
* shared scripts from test-infra live in //vendor/github.com/knative/test-infra/scripts;
* created `//hack/update-deps.sh` to properly update deps, including test-infra;
* all bash scripts were updated to use the vendored scripts;
Part of knative/test-infra#30.
Scott Nichols
Adriano Cunha