delete all ignored files

This will remove all ignored files effectively resetting your current state to a fresh clone. git clean -dfX

committed to wrong branch

This will revert the commit, but put the committed changes back into your index. Then you can switch to the correct branch and make the commit again:
git reset --soft HEAD^

amend last commit

Allows you to change the message or contents of your last (unpushed) commit:
git amend

If you accidentally amend you can undo it with:
git reset --soft HEAD@{1}

Git doesn't actually "replace" the old commit. It writes a new commit which also points from the old commits parent. Then it updates your branch to follow the new commit. So where is the old "overwritten" commit?
git log --graph --oneline --reflog

For older commits you can arbitrarily change them quickly with git rebase.