t/t5333-pseudo-merge-bitmaps.sh: demonstrate empty pseudo-merge groups

t/t5333-pseudo-merge-bitmaps.sh: demonstrate empty pseudo-merge groups

Demonstrate that it is possible to generate empty pseudo-merge commits
in certain cases.

In the below instance, we generate one non-empty pseudo-merge
(containing commit "base"), and one empty pseudo-merge group
(corresponding to the unstable commits within that group).

(In my testing, the pseudo-merge machinery seems to handle empty groups
just fine, but generating them is pointless as they carry no
information.)

This commit (introducing a deliberate "test_expect_failure") is split
out from the actual fix (which will appear in the following commit) to
demonstrate that the failure is correctly induced.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>