Separate Git branch references from the Workflow concept

[MarkRobbo]

Currently workflows can be added by branch (eg master) or commit ID (eg aaf4c61ddcc5e8a2dabede0f3b482cd9aea9434d). These are added as distinct workflows even if they refer to the same thing at a moment of time.

This is due to the GitDetails object used as the retrievedFrom ID of the workflow being a combination of the repository URL, branch (or commit ID), path and packed ID.

Ideally the concept of a branch reference or tag would be a higher level layered concept which points to a commit ID similar to the Git internals, so that if a branch leaves a commit behind, it still exists in the database and permalinks will continue to exist.

For now, the workaround to this will be to create an additional workflow for the commit ID when a branch moves on