[WIP[JENKINS-18574] Fixes for multiple locations in same repo

# Do not merge (yet).

This is an improved solution to JENKINS-18574 that does not suffer from JENKINS-23146.

It also enhances the "changes" view to show both the path in the repo, as well as the path in the workspace.

**This is not yet finished**, is it only shows one workspace path per file in the repo (although it could have been checked out multiple times to multiple paths in the workspace). Only tested on Linux master as well.