Create script to "vendor" test-infra/scripts (#2068)
Create script to "vendor" test-infra/scripts (#2068)

* Create script to "vendor" test-infra/scripts

Tracks the contents of test-infra/scripts into each repo at
scripts/test-infra directory.

The script provides both installation assistance (with --first-time
command-line argument) as well as its usual function of updating
scripts/test-infra directory and is intended to be called by
hack/update-deps.sh in each repo. The --branch option lets you choose
a particular branch (instead of master) if desired.

* Fix shellcheck complaint

* Add notes and changes from discussion

Even though this is the route we will probably not take

* A little bit of cleanup

* Let build tests pass by requiring --update

* Little fixes

* Print message when remote already set
knative/test-infra,knative/test-infra,knative/test-infra