[Bug] incomplete defence agains creating a branch named HEAD

So git branch doesn't like to create a branch named HEAD

$ git branch HEAD fatal: it does not make sense to create 'HEAD' manually

But, you can trick it into doing so anyway:

$ git branch @
$ git branch -a
  HEAD
* master

At which point git status becomes a bit confused:

$ git status warning: refname 'HEAD' is ambiguous. warning: refname 'HEAD' is ambiguous. On branch master nothing to commit, working directory clean

Oh, and git checkout will accept either HEAD or @ to create a branch with the name HEAD anyway:

$ git checkout -b HEAD Switched to a new branch 'HEAD'

$ git checkout -b @ Switched to a new branch 'HEAD'

imho none of these should create a branch named HEAD, but should do what 'git branch HEAD' does and fail with a sensible error message.

All these shenanigans came up when trying to help an user who is mirroring a mercurial repo that has a branch named '@'. Whether or not git should allow branches named '@' I don't have an opinion on, I know '@' is pretty special when dealing with refs.