Another day, another discovery of classic tool that does exactly what I want, this time it's git worktree for the rescue:

I was working on updating a #cpp project to a newer #Qt version, but wanted to keep comparing to a #git tag from some time ago. The problem is that checking out a different commit changes all the files as far as #CMake is concerned, and everything builds from scratch. If only there was a way to have another copy of the same repo without cloning...