completion: add tests showing subpar switch/checkout --orphan logic

completion: add tests showing subpar switch/checkout --orphan logic

Similar to -c/-C, --orphan takes an argument which is the branch name to
use. We ought to complete this branch name using similar rules as to how
we complete new branch names for -c/-C and -b/-B. Namely, limit the
total number of options provided by completing to the local branches.

Additionally, git switch --orphan does not take any start point and will
always create using the empty-tree. Thus, after the branch name is
completed, git switch --orphan should not complete any references.

Add test cases showing the expected behavior of --orphan, for both the
argument and starting point.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>