git-branch -D: make it work even when on a yet-to-be-born branch

This makes "git branch -D other_branch" work even when HEAD
points at a yet-to-be-born branch.

Earlier, we checked the HEAD ref for the purpose of "subset"
check even when the deletion was forced (i.e. not -d but -D).
Because of this, you cannot delete a branch even with -D while
on a yet-to-be-born branch.

With this change, the following sequence that now works:

mkdir newdir && cd newdir
git init-db
git fetch -k $other_repo refs/heads/master:refs/heads/othre
# oops, typo
git branch other othre
git branch -D othre

Signed-off-by: Junio C Hamano <junkio@cox.net>