release-toolkit code refactoring (reorganize methods and files)

[AliSoftware]

Refactor parts of the release-toolkit code in order to:

• Put some of the methods in better places – especially a lot of methods that are in *_git_helper are not really related to git per se (and are probably only there because they end up pushing things at the of of what they actually do, which is only part of their action)
• Make it more testable where needed

Note: We'll very likely work on the YARD doc issue #192 in parallel of working on this.

[AliSoftware]

Related: https://github.com/wordpress-mobile/release-toolkit/issues/255