t/t6500-gc.sh: add additional test cases

t/t6500-gc.sh: add additional test cases

In the last commit, we refactored some of the tests in t6500 to make
clearer when cruft packs will and won't be generated by `git gc`.

Add the remaining cases not covered by the previous patch into this one,
which enumerates all possible combinations of arguments that will
produce (or not produce) a cruft pack.

This prepares us for a future commit which will change the default value
of `gc.cruftPacks` by ensuring that we understand which invocations do
and do not change as a result.

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