Store the revision of the user's code that's been checked out.
Store the revision of the user's code that's been checked out.

This adds a .user-rev file alongside the user's code so they can see
which version they've got checked out. This is similar to the existing
.pyenv-rev that gets inclduded by the pyenv wrapper.

jmorse/cyanide,jmorse/cyanide,jmorse/cyanide,jmorse/cyanide