t5516: drop unnecessary subshell and command invocation

To create its "expect" file, this test pipes into `sort` the output of `git for-each-ref` and a copy of that same output but with a minor textual transformation applied. To do so, it employs a subshell and commands `cat` and `sed` even though the same result can be accomplished by `sed` alone (without a subshell). Signed-off-by: Eric Sunshine <sunshine@sunshineco.com> Reviewed-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>