When git am --skip is run, git am will copy HEAD's tree entries to the
index with "git reset HEAD". However, on an unborn branch, HEAD does not
point to a tree, so "git reset HEAD" will fail.
Fix this by treating HEAD as en empty tree when we are on an unborn
branch.
Signed-off-by: Paul Tan <pyokagan@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>