t5505: modernize and simplify hard-to-digest test

This test uses a subshell within a subshell but is formatted in such a
way as to suggests that the inner subshell is a sibling rather than a
child, which makes it difficult to digest the test's structure and
intent.

Worse, the inner subshell performs cleanup of actions from earlier in
the test, however, a failure between the initial actions and the cleanup
will prevent the cleanup from taking place.

Fix these problems by modernizing and simplifying the test and by using
test_when_finished() for the cleanup action.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5505-remote.sh

@@ -348,17 +348,13 @@ URL: $(pwd)/one
 EOF
 
 test_expect_success 'prune --dry-run' '
-	(
-		cd one &&
-		git branch -m side2 side) &&
+	git -C one branch -m side2 side &&
+	test_when_finished "git -C one branch -m side side2" &&
 	(
 		cd test &&
 		git remote prune --dry-run origin >output &&
 		git rev-parse refs/remotes/origin/side2 &&
 		test_must_fail git rev-parse refs/remotes/origin/side &&
-	(
-		cd ../one &&
-		git branch -m side side2) &&
 		test_i18ncmp expect output
 	)
 '