PR task in 0. Git Basics subtopic

This issue is a response to Ivan Tytarenko's request.

Subtopic **0. Git Basics** has a task (PR to Kottans/mock-repo) that for new GitHub users hard to complete without watching a tutorial from subtopic **2. GitHub and Collaboration**.
**0. Git Basics** has information about using Git in the local environment, but the task implicates that a student knows about forking and PRs.

As a solution, I can propose to combine subtopics **0. Git Bash** and **2. GitHub and Collaboration** with all tasks that they have.

Review subtopics 0. Git Basics and 2. GitHub and Collaboration, including the task and its PR to Kottans/mock-repo. Compare the prerequisites and decide whether to combine the subtopics and all their tasks; done means the Git Basics task can be completed by new GitHub users without relying on the later tutorial.

Written by the indexing model from the issue text.