How to leave only one sub-directory in a git repository

leaving_only_one_subdirectory_in_git_repo.md

The following command leaves only content of my/sub/dir in a master branch:

git filter-branch --subdirectory-filter my/sub/dir --prune-empty master

as a side effect, my/sub/dir becomes a root directory of a repository.

Full documentation on git filter-branch command could be found here: https://git-scm.com/docs/git-filter-branch