Cumulative changes for workspaces

Idea from original Workspaces issue gitlab-ce#47062:

Subsequent jobs get all cumulative changes from previous jobs. (e.g., two parallel jobs won't see each other's changes, but the downstream job receives the union of their changes)

This would allow users to easily have multiple jobs in a stage 'add' content to the workspace. Then jobs downstream (later stages) would be able to see BOTH previous job's changes - even if they ran in parallel.