Explain the key difference in how `git revert` and `git reset --hard` impact the permanent history of the repository.

The key difference in how `git revert` and `git reset --hard` impact the permanent history of the repository lies in their approach to undoing changes: `git revert` creates new history, while `git reset --hard` rewrites or effectively deletes existing history.

`git revert` is a non-destructive operation. When you use `git revert <commit>`, Git creates a brand-new commit, known as a revert commit. This revert commit introduces changes that precisely undo the modifications made in the specified original commit, but the original commit itself, along with all subsequent commits, remains intact in the project's history. The repository's history, which is the sequence of all commits, retains a complete record of both the original change and its undoing. This process adds to the history rather than altering it. For example, if commit C introduced a bug, `git revert C` would create a new commit D that removes the bug's changes, resulting in a history where C and D both exist, allowing you to trace exactly what happened.

Conversely, `git reset --hard <commit>` is a destructive operation that rewrites history. It moves the current branch pointer, which is the reference to the latest commit on that branch, to the specified target commit. Critically, it also forces the working directory, which is the actual files you see and edit, and the staging area, which holds changes prepared for the next commit, to match the state of that target commit. Any commits that existed after the target commit on the original branch are effectively removed from that branch's history. These commits are no longer part of the linear sequence of changes for that branch and are typically lost from the repository's history for collaborators if pushed remotely. For instance, if you have commits A, B, C, and D on your branch (D being the latest) and you execute `git reset --hard B`, commits C and D are removed from the branch's history, and your working directory and staging area will revert to the state of commit B. This means the history of your branch has been rewritten to end at commit B.