t3200: verify "branch --list" sanity when rebasing from detached HEAD

t3200: verify "branch --list" sanity when rebasing from detached HEAD

"git branch --list" shows an in-progress rebase as:

  * (no branch, rebasing <branch>)
    master
    ...

However, if the rebase is started from a detached HEAD, then there is no
<branch>, and it would attempt to print a NULL pointer. The previous
commit fixed this problem, so add a test to verify that the output is
sane in this situation.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Kaartic Sivaraam <kaartic.sivaraam@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>