`danger local` git diff error

Hi,

When running `danger local --dangerfile dangerfile.js` I am getting:

```
Could not get diff from git between master and HEAD
{ Error: stdout maxBuffer exceeded
at Socket.onChildStdout (child_process.js:325:14)
at Socket.emit (events.js:160:13)
at addChunk (_stream_readable.js:269:12)
at readableAddChunk (_stream_readable.js:252:13)
at Socket.Readable.push (_stream_readable.js:213:10)
at Pipe.onread (net.js:599:20) cmd: 'git diff master...HEAD' }
```

For branch structure:

master -> develop -> build/danger

Start by reproducing `danger local --dangerfile dangerfile.js` with the reported master → develop → build/danger branch structure and inspect the failing `git diff master...HEAD` command. Done means local mode obtains the diff without the `stdout maxBuffer exceeded` error.

Written by the indexing model from the issue text.