You can refer to the GitHub page "Duplicating a repository"
It uses:
git clone --mirror: to clone every references (commits, tags, branches)git push --mirror: to push everything
That would give:
git clone --mirror https://bitbucket.org/exampleuser/repository-to-mirror.git
# Make a bare mirrored clone of the repository
cd repository-to-mirror.git
git remote set-url --push origin https://github.com/exampleuser/mirrored
# Set the push location to your mirror
git push --mirror