Make test "using invalid commit with -C" more strict

Make test "using invalid commit with -C" more strict

In the test 'using invalid commit with -C' git-commit would have failed
even if the -C option had been given the correct commit, as there was
nothing to commit. Pass --allow-empty to make sure it would make a commit,
were there no issues with the argument given to the -C option.

Signed-off-by: Kacper Kornet <redacted>
Signed-off-by: Junio C Hamano <redacted>