Fix --base option for danger local cli

[peterjgrainger]

Fixed #496

[peterjgrainger]

I see the test failing, i'll try to recreate locally

[peterjgrainger]

@orta A previous build fails on the same issue https://travis-ci.org/danger/danger-js/jobs/336976131, I think this test fails if git log master...HEAD has no history i.e. HEAD === master

Let me know if I need to change anything. I believe everything is working as it is supposed to.

[orta]

Yeah, that's something I've wanted to fix but haven't (been working on danger/peril#159 for the last few weeks straight) I was going to add a check if head === master just exit with 0 and give a log saying "doing nothing"

[orta]

Looks 👍 regardless :D