t7406: fix call that was failing for the wrong reason

t7406: fix call that was failing for the wrong reason

A test making use of test_must_fail was failing like this:
  fatal: ambiguous argument '|': unknown revision or path not in the working tree.
when the intent was to verify that a specific string was not found
in the output of the git diff command, i.e. that grep returned
non-zero.  Fix the test to do that.

Signed-off-by: Elijah Newren <redacted>
Signed-off-by: Junio C Hamano <redacted>