Spec coverage

[begedin]

• Asana task: Cover code gem with specs

  Description

The objective of this PR is to cover the entire codebase with specs.

Progress

• [x] code/system/os
• [x] code/system
• [x] code/git
• [x] code/github_api
• [x] code/branch
• [x] code/repository
• [x] code/pull_request
• [x] code/file_list

[begedin]

I think this is about as covered as it can be. Marked "awaiting review"