Detect default branch change in repository GitHub event

Summary:
The repository GitHub event has now in the API a new action "edited",
for example when the repository default branch is changed.

This change assigns to a default branch change payload a new event
"default_branch" overriding the GitHub "repository" one, to avoid
raising too much the complexity of RepositoryEvent.

Fixes T1729.

Test Plan: Unit test provided.

Reviewers: DorianWinty

Reviewed By: DorianWinty

Maniphest Tasks: T1729

Differential Revision: https://devcentral.nasqueron.org/D2680