t/t5318-commit-graph.sh: test generation zero transitions during fsck

t/t5318-commit-graph.sh: test generation zero transitions during fsck

The second test called "detect incorrect generation number" asserts that
we correctly warn during an fsck when we see a non-zero generation
number after seeing a zero beforehand.

The other transition (going from non-zero to zero) was previously
untested. Test both directions, and rename the existing test to make
clear which direction it is exercising.

Signed-off-by: Taylor Blau <redacted>
Signed-off-by: Junio C Hamano <redacted>