I'd like to create a new master branch from an existing tag. Say I have a tag v1.0. How to create a new branch from this tag?

Wow, that was easier than I thought:

git checkout -b newbranch v1.0

If running the above command you get an error

warning: refname 'v1.0' is ambiguous.
fatal: ambiguous object name: 'v1.0'

this is because you also have a branch with name v1.0. In this case specify tag/ prefix:

git checkout -b newbranch tags/v1.0