git-p4: ensure complex branches are cloned correctly

When importing a branch from p4, git-p4 searches the history of the parent branch for the branch point. The test for the complex branch structure ensures all files have the expected contents, but doesn't examine the branch structure. Check for the correct branch structure by making sure that the initial commit on each branch is empty. This ensures that the initial commit's parent is indeed the correct branch-off point. Signed-off-by: Joachim Kuebart <joachim.kuebart@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>