t7500: make each piece more independent

t7500: make each piece more independent

These tests prepare the working tree & index state to have something
to be committed, and try a sequence of "test_must_fail git commit".
If an earlier one did not fail by a bug, a later one will fail for
a wrong reason (namely, "nothing to commit").

Give them "--allow-empty" to make sure that they would work even
when there is nothing to commit by accident.

Signed-off-by: Junio C Hamano <redacted>
Signed-off-by: Taylor Blau <redacted>
Signed-off-by: D. Ben Knoble <redacted>
Signed-off-by: Junio C Hamano <redacted>