git: switch branch without detaching head

# first time: make origin/branchname locally available as localname
git checkout -b localname origin/branchname

# othertimes
git checkout localname

git push origin

branch -a

git clone git@github.com:abc/def.git
cd def

git branch --track experimental origin/experimental
git checkout experimental

git push

$ git branch experimental origin/experimental

$ git checkout experimental

$ git checkout -b experimental origin/experimental