git p4: test sync/clone --branch behavior

git p4: test sync/clone --branch behavior

Add failing tests to document behavior when there are multiple p4
branches, as created using the --branch option.  In particular:

Using clone --branch populates the specified branch correctly, but
dies with an error when trying to checkout master.

Calling sync without a master branch dies with an error looking for
master.  When there are two or more branches, a sync does
nothing due to branch detection code, but that is expected.

Using sync --branch to try to update just a particular branch
updates no branch, but appears to succeed.

Signed-off-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>