t2018: teach do_checkout() to accept `!` arg

We are running `test_must_fail do_checkout`. However,
`test_must_fail` should only be used on git commands. Teach
do_checkout() to accept `!` as a potential first argument which will
cause the function to expect the "git checkout" to fail.

This increases the granularity of the test as, instead of blindly
checking that do_checkout() failed, we check that only the specific
expected invocation of git fails.

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

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