git.py should not import MepoState (from state.py)

[pchakraborty]

Currently GitRepository uses MepoState to get the full local path of the fixture, in order to construct the full local path of a sub-component, which are specified in the registry in relative form. Maybe there is a better way to get the full local path of the fixture that doesn't require dependence on MepoState.

One consequence of git.py's dependence on MepoState is that we need two separate copies of get_current_remote_url - one in git.py and the other in component.py, to avoid circular dependency. Once #312 is implemented, the need for get_current_remote_url goes away, but it will still be good to find a way to remove this dependence.

[pchakraborty]

Resolved in #316