t0001: use test_must_fail

t0001: use test_must_fail

We've hand-rolled several "if" statements looking for
failures. We can use test_must_fail here, which is shorter
and more robust.

Note that we modify the commands slightly (to use "git init
foo" rather than "cd foo && git init") to avoid dealing with
a subshell, but this should not affect the outcome.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>