Allow merge commits in detached head state

[waldyrious]

In http://onlywei.github.io/explain-git-with-d3/#freeplay:

Do whatever you want in this free playground.

$ git checkout HEAD
$ git commit
$ git commit
# generated hash: d525e9e...
$ git checkout master
$ git checkout -b branch1
$ git commit
$ git commit
git checkout d525e9e
$ git merge branch1
Not a good idea to make commits while in a detached HEAD state.

I thought I was allowed to do anything here? 😄