Consistency for Git operations

[iangmaia]

In release-toolkit, we use a mix of Action.sh('git'), `git [etc]` and Fastlane actions such as push_to_git_remote to perform Git operations.

We should look into standardizing these Git operations. A suggestion is to try and use the Git gem instead (see #570 (comment)).

[mokagio]

configure_helper.rb is one of the files where we use Git under the hood and that could do with improvements.

I took a stab at rewriting one method from raw git commands to using the Git gem as part of #410. There are some unit tests, too, which we could build on top of for future refactors.