GetChanges is returning files that didn't change in the PR

[dpordomingo]

Given this history:

* 7450de7 (origin/master) Change 'README.md' file
|
| * dd8c39e (fork/candidate) Change 'golang.go' file
| |
| /   
* 83f7e4d
| 
* 7ccf9be

For a pull request to merge fork/candidate against origin/master, GetChanges rpc is returning golang.go and README.md.

I guess it's because DataService is fetching changes with kind of:
git diff 7450de7..dd8c39e
that returns all differences between 7450de7 and dd8c39e (what is wrong)

instead of doing:
git diff 7450de7...dd8c39e
that returns the differences between common ancestor (83f7e4d) and dd8c39e, representing the changes made by the branch being pullrequested.

Keeping this, forces the analyzer to avoid posting comments about files that were not modified in the PR being reviewed (ie. README.md), because otherwise GitHub API will return an unhandled Path is invalid error:

{
  "message": "Unprocessable Entity",
  "errors": []github.Error{}[
    github.Error{
      Resource:"Review",
      Field:"",
      Code:"custom",
      Message:"Path is invalid"
    }
  ],
}

(described by #487)

[dpordomingo]

I think GitHub API is returning an unhandled Path is invalid error due to google/go-github#540

[carlosms]

Let's make sure we include a a test with this scenario in https://github.com/src-d/lookout-test-fixtures