[Bug] incomplete defence agains creating a branch named HEAD

[Dennis Kaarsemaker <dennis@kaarsemaker.net>]

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.

-- 
Dennis Kaarsemaker
www.kaarsemaker.net