Undo

gistfile1.txt

git rm -r --cached 

git push origin +dd61ab32^:master

git reset HEAD^ --hard

git push origin -f

git reset HEAD^

Undo a commit and Redo

$ git commit -m "Something terribly misguided"              (1)
$ git reset HEAD~                                           (2)
<< edit files as necessary >>                               (3)
$ git add ...                                               (4)
$ git commit -c ORIG_HEAD                                   (5)