Some test repositories are not correctly removed after the tests
did run. Fix by introducing a function that is to be used for
cleaning up temporary test repositories.
When we create a commit and tell the library to update a ref, we need to
make sure to pass the current tip.
This is what we should have been doing in a function called
'upateReadme()' anyway. The existing code creates a new root commit,
which is not an update.
Patrick Steinhardt
Mark Probst
Carlos Martín Nieto
Jesse Ezell
Vicent Marti