mergetool--lib: remove no-op assignment to $status from setup_user_tool

Even though setup_user_tool assigns the exit status from "eval $merge_tool_cmd" to $status, the variable is overwritten by the function it calls next, check_unchanged, without ever getting looked at by anybody. And "return $status" at the end of this function returns the value check_unchanged assigned to it (which is the same as the value the function returns). Which makes the assignment a no-op. Remove it. Signed-off-by: Junio C Hamano <gitster@pobox.com>