t1404: remove "prefix" argument to test_update_rejected

t1404: remove "prefix" argument to test_update_rejected

The tests already set a variable called prefix and passed its value as
the first argument to this function. The old argument handling was
overwriting the global variable with its same value rather than creating
a local variable.

So change test_update_rejected to refer to the global variable rather
than taking the prefix as an argument.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>