Unexpected behaviour calling diffForFile on added or renamed file

[chris48s]

danger version: 3.1.7

Looking at #369 I think the intended behaviour is that it should be possible to get a diff for a created file, but I am observing different behaviour. Maybe there has been a regression? For example, if I declare the dangerfile:

const { danger, fail, message, warn } = require('danger');

danger.git.diffForFile('dangerfile.flow.js').then(function(diff) {
  message('foo');
});

I observe the following:

$ npm run danger pr "https://github.com/danger/danger-js/pull/518"

> test-app@ danger [path]
> danger "pr" "https://github.com/danger/danger-js/pull/518"

Starting Danger PR on danger/danger-js#518
Error:  { HttpError: {"message":"Not Found","documentation_url":"https://developer.github.com/v3/repos/contents/#get-contents"}
    at IncomingMessage.response.on ([path]/request.js:73:18)
    at emitNone (events.js:111:20)
    at IncomingMessage.emit (events.js:208:7)
    at endReadableNT (_stream_readable.js:1055:12)
    at _combinedTickCallback (internal/process/next_tick.js:138:11)
    at process._tickCallback (internal/process/next_tick.js:180:9)
  name: 'HttpError',
  code: 404,
  status: 'Not Found',
  headers: 
   { [truncated] }

## Failures
node` failed.
## Markdowns
### Log

Danger: Failing the build, there is 1 fail.

Similarly, if I declare the dangerfile:

const { danger, fail, message, warn } = require('danger');

danger.git.diffForFile('source/dsl/cli-args.ts').then(function(diff) {
  message('foo');
});

I observe the following:

$ npm run danger pr "https://github.com/danger/danger-js/pull/518"

> test-app@ danger [path]
> danger "pr" "https://github.com/danger/danger-js/pull/518"

Starting Danger PR on danger/danger-js#518
Error:  { HttpError: {"message":"Not Found","documentation_url":"https://developer.github.com/v3/repos/contents/#get-contents"}
    at IncomingMessage.response.on ([path]/request.js:73:18)
    at emitNone (events.js:111:20)
    at IncomingMessage.emit (events.js:208:7)
    at endReadableNT (_stream_readable.js:1055:12)
    at _combinedTickCallback (internal/process/next_tick.js:138:11)
    at process._tickCallback (internal/process/next_tick.js:180:9)
  name: 'HttpError',
  code: 404,
  status: 'Not Found',
  headers: 
   { [truncated] }

## Failures
node` failed.
## Markdowns
### Log

Danger: Failing the build, there is 1 fail.

If I attempt to get a diff of any of the modified files, for example:

const { danger, fail, message, warn } = require('danger');

danger.git.diffForFile('source/dsl/DangerDSL.ts').then(function(diff) {
  message('foo');
});

then the output is:

$ npm run danger pr "https://github.com/danger/danger-js/pull/518"

> test-app@ danger [path]
> danger "pr" "https://github.com/danger/danger-js/pull/518"

Starting Danger PR on danger/danger-js#518
## Messages
foo
Danger: Passed, found only messages.

as expected.