Revert "Revert "Update default src/ports branches"

This was mistakenly reverted because of confusion with
GIT_TAG and --depth 1 causing tagged checkouts to fail.

We need to restore this becuase the branches have diverged
between master and releng/9.1.0

This reverts commit 2f3aa54748247881bd5cc37b522ef460f62d3497.

Conflicts:

	build/do_build.sh