git-p4: Verify detection of "empty" branch creation

git-p4: Verify detection of "empty" branch creation

Current implementation of new branch parent detection works on the
principle that the new branch is a complete integration, with no
changes, of the original files.
This test shows this deficiency in the particular case when the new
branch is created from a subset of the original files.

Signed-off-by: Vitor Antunes <vitor.hda@gmail.com>
Acked-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>