Fix error in setup of basic-branching kata
Fix error in setup of basic-branching kata

This commit will fix an error in the setup.sh of the
basic-branching kata.
After `git init` the setup script tried to checkout the already
checked out master branch. that showed an error:
`error: pathspec 'master' did not match any file(s) known to git`

Closes #117

praqma-training/gitkatas,praqma-training/gitkatas