WIP: Git Undo tutorial

Outline of Undo changes in Git tutorial as in blog-posts#97 (closed) was given some more details, along with general structure and style of the tutorial.

In general all outline topics are covered and focus is really on Undoing the changes instead of blind usage of commands.

Todo

• link tip's n trick's in first paragraph (https://about.gitlab.com/2016/12/08/git-tips-and-tricks/)
• re-check for missing commands from outline (maybe make them more visible)
• re-check author description
• re-format the structure (heading 4's are small, yet they are probably more important than others)
• add image to the post (no idea what to put)

@marcia

Closes blog-posts#97 (closed)