Create dev branch from master branch
git checkout -b dev
Push branch to remote origin
git push origin dev
Make dev your default branch (in Bitbucket)
Delete master branch (in Bitbucket and local)
git branch -d master
Create new master branch without history and commit it
git checkout -b master <SHA from initial commit>
git merge --squash <branch>
git commit -m "your merge message"
Push new master
git push origin master
Set master to your default branch (in Bitbucket)
Add Github as remote target to master branch
git remote add -t master github_origin url
Push to Github
git push github_origin master
git checkout master
git merge --squash <feature branch>
git commit -m "your message"