Mention git merge --abort and git rebase --abort

[theOperand]

In my experience, the most common "getting stuck" situation for novice git users is getting stuck doing a rebase or a merge with conflicts.

In these situations, doing git merge --abort or git rebase --abort will put you right back where you were before executing the merge or rebase, and is the simplest way to recover from accidental bad merges or rebases.

I think the page should probably mention these commands because they very simple and work in the most common situations.

[jonlandrum]

we need a whole section on merge/pull requests