completion: add tests showing subpar -c/-C startpoint completion

completion: add tests showing subpar -c/-C startpoint completion

When using the branch creation argument for git switch or git checkout,
-c/-C or -b/-B, the commands operate in a different mode: `git switch -c
<branch> <some-reference>` means to create a branch named <branch> at
the commit referred to by <some-reference>.

When completing the start-point, we ought to always complete all valid
references.

Add tests for the completion of the start-point to -c/-C and -b/-B.

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