add example on git revert + differenciate from git restore

[avallecam]

• add revert demo https://github.com/Bisaloo/revertdemo/
• added in https://epiverse-trace.github.io/git-rstudio-basics/05-history.html#these-commands-are-so-similar
• linked to https://www.atlassian.com/git/tutorials/undoing-changes

keypoints:

• Once changes have been committed they are generally permanent
• Use git checkout to move around and review the commit history
• git revert is the best tool for undoing shared public changes
• git reset is best used for undoing local private changes
• git clean for undoing uncommitted changes
• use git restore to discard changes in working directory [this is the revert button in RStudio - misunderstanding]

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)