Rebasing onto a new initial commit

gistfile1.sh

git checkout master
git checkout -b rebaser
git checkout master
git update-ref -d refs/heads/master
git rm -rf $EVERYTHING_EXCEPT_GITIGNORE # figuratively
git commit -m 'initial commit'

# this will remove the commits off of the remote (to be GC'd at some indeterminate point)
git push --force origin master

# now, rebase onto this new, sparse, initial commit
git checkout rebaser
git rebase master # use -i to reword the formerly-initial commit

# assuming you want to make a PR
git push -u origin rebaser

# then, if you want to bring it all back
git checkout master
git reset --hard rebaser
git push --force origin master