Enable git fast forward pull

1) Enables fast forward pull by default.
2) Fixes a bug in non-fast-forward pull.


@devoncarew PTAL thanks.
And thinking about this, our error message should give the user some hint as to what they can do to work around it. So something like:

    Merge conflicts detected. Chrome Dev Editor does not currently support non-fast forward pulls.
    As a work-around you can merge your changes on the remote server and pull those merged changes in.