t6300: refactor %(trailers) tests

We currently have one test for %(trailers) in `git-for-each-ref(1)`,
through "%(contents:trailers)". In preparation for more, let's add a few
things:

  - Move the commit creation step to its own test so that it can be
  re-used.

  - Add a non-trailer to the commit's trailers to test that non-trailers
  aren't shown using "%(trailers:only)".

  - Add a multi-line trailer to ensure that trailers are unfolded
  correctly using "%(trailers:unfold)".

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6300-for-each-ref.sh

@@ -605,18 +605,25 @@ test_expect_success 'do not dereference NULL upon %(HEAD) on unborn branch' '
 cat >trailers <<EOF
 Reviewed-by: A U Thor <author@example.com>
 Signed-off-by: A U Thor <author@example.com>
+[ v2 updated patch description ]
+Acked-by: A U Thor
+  <author@example.com>
 EOF
 
-test_expect_success 'basic atom: head contents:trailers' '
+
+test_expect_success 'set up trailers for next test' '
 	echo "Some contents" > two &&
 	git add two &&
-	git commit -F - <<-EOF &&
+	git commit -F - <<-EOF
 	trailers: this commit message has trailers
 
 	Some message contents
 
 	$(cat trailers)
 	EOF
+'
+
+test_expect_success 'basic atom: head contents:trailers' '
 	git for-each-ref --format="%(contents:trailers)" refs/heads/master >actual &&
 	sanitize_pgp <actual >actual.clean &&
 	# git for-each-ref ends with a blank line