$ git checkout --orphan NEWBRANCH
$ git rm -rf .
--orphan
creates a new branch, but it starts without any commit. After running the above command you are on a new branch "NEWBRANCH", and the first commit you create from this state will start a new history without any ancestry.
You can then start adding files and commit them and they will live in their own branch. If you take a look at the log, you will see that it is isolated from the original log.
@Ogollah I know I'm late to the party but, yes you can merge master into the new banch. You can even fast forward the new branch to master's state (which will basically move the new branch's ref next to master's)