Separate Git branch references from the Workflow concept

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_