We do not like "HEAD" as a new branch name

We do not like "HEAD" as a new branch name

This makes git-check-ref-format fail for "HEAD". Since the check is only
executed when creating refs, the existing symbolic ref is safe.

Otherwise these commands, most likely are pilot errors, would do
pretty funky stuff:

	git checkout -b HEAD
	git pull . other:HEAD

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>