t9801: do not overuse test_must_fail

t9801: do not overuse test_must_fail

test_must_fail is to make sure a program we can potentially break during
the course of updating git itself exits with a non-zero status in a clean
and controlled way.

When we expect a non-zero exit status from the commands we use from the
underlying platform in tests, e.g. making sure a string "error: " does not
appear in the output by running "grep 'error: '", just use "! grep" for
readability. It is not like we will try to update Git and suddenly 'grep'
we use from the system starts segfaulting.

Signed-off-by: Junio C Hamano <gitster@pobox.com>