Use 'worktree' feature of git to maintain parallel branches

[freephile]

We need to maintain 'main' (using e.g. REL1_34) while we also work on a development branch for REL1_35 (aka 'dev'); AND be able to merge feature or bugfix branches (based off 'main') into 'dev'. The 'worktree' feature of git seems like the right approach to make this easier. Commentary at https://spin.atomicobject.com/2016/06/26/parallelize-development-git-worktrees/