Fix GetChanges issue with changes in both sides

[dpordomingo]

fix #486

depends on src-d/go-git#1097
depends on src-d/go-git-fixtures#13
supersedes #530

This PR fix #486, so when an analyzer reviews the PR of get-changes-from-outdated-pr-candidate against get-changes-from-outdated-pr, it should not fail (as it's currently happening here)

* 3c9686a (get-changes-from-outdated-pr) Deletes something in 'golang.go' file
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| * 5de771f (get-changes-from-outdated-pr-candidate) Creates a new 'javascript.js' file
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| /   
* 44034c9 (common-ancestor) creates a new 'golang.go' file

breaking changes

DataService.GetChanges is currently returning the differences between base and head, so when this is merged, it will return the differences between git merge-base base head and head.

In case any analyzer wants to get the changes between base and head it should configure the ChangeRequest.TwoDotsMode, as true as described by src-d/lookout-sdk#76

[dpordomingo]

Travis is failing at Generated code stage https://travis-ci.com/src-d/lookout/jobs/187599618 because vendor is currently hardcoded at 9a4e7e2 (because it has not been yet merged on go-git nor go-git-fixtures)