git-p4: add failing test for name-rev rather than symbolic-ref

git-p4: add failing test for name-rev rather than symbolic-ref

Using name-rev to find the current git branch means that git-p4
does not correctly get the current branch name if there are
multiple branches pointing at HEAD, or a tag.

This change adds a test case which demonstrates the problem.
Configuring which branches are allowed to be submitted from goes
wrong, as git-p4 gets confused about which branch is in use.

This appears to be the only place that git-p4 actually cares
about the current branch.

Signed-off-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>