git-p4: ensure complex branches are cloned correctly

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>