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>