Change the response type of "createFile" and "updateFile"

[jmini]

Fixes #893

The response JSON of "createFile" and "updateFile" is:

{
  "file_path": "app/project.rb",
  "branch": "master"
}

Documentation:

• https://docs.gitlab.com/ee/api/repository_files.html#create-new-file-in-repository
• https://docs.gitlab.com/ee/api/repository_files.html#update-existing-file-in-repository

This PR introduce a new type RepositoryFileResponse that corresponds to this JSON.

Since 6.0.0 is a new major version, I think it is fine to do this breaking change.