Refactor `Queue` class

[josecelano]

I think we can implement a couple of improvements in the Queue class.

Refactors:

• Extract class GitRepoDir
• Extract class GitRepo
• Extract class CommittedMessageLog (collection of CommittedMessage)
• Move CommitOptions to a class attribute
• Handle empty or not initialized git repo gracefully
• Set attributes as private
• Refactor createInitializedTempGitDir
• Add missing tests for Queue class: guard clauses, ...