t3415: increase granularity of test_auto_{fixup,squash}()

t3415: increase granularity of test_auto_{fixup,squash}()

We are using `test_must_fail test_auto_{fixup,squash}` which would
ensure that the function failed. However, this is a little ham-fisted as
there is no way of ensuring that the expected part of the function
actually fails.

Increase the granularity by accepting an optional `!` first argument
which will check that the rebase does not squash in any commits by
ensuring that there are still 4 commits. If `!` is not provided, the old
logic is run.

This patch may be better reviewed with `--ignore-all-space`.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>