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 <[email protected]>
Reviewed-by: Elijah Newren <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: sunshineco

t/t5516-fetch-push.sh

@@ -1316,10 +1316,7 @@ test_expect_success 'fetch follows tags by default' '
 		git pull ../testrepo main &&
 		git tag -m "annotated" tag &&
 		git for-each-ref >tmp1 &&
-		(
-			cat tmp1
-			sed -n "s|refs/heads/main$|refs/remotes/origin/main|p" tmp1
-		) |
+		sed -n "p; s|refs/heads/main$|refs/remotes/origin/main|p" tmp1 |
 		sort -k 3 >../expect
 	) &&
 	git init dst &&