t7510: invoke git as part of &&-chain

If `git commit-tree HEAD^{tree}` fails on us and produces no output on
stdout, we will substitute that empty string and execute `git tag
ninth-unsigned`, i.e., we will tag HEAD rather than a newly created
object. But we are lucky: we have a signature on HEAD, so we should
eventually fail the next test, where we verify that "ninth-unsigned" is
indeed unsigned.

We have a similar problem a few lines later. If `git commit-tree -S`
fails with no output, we will happily tag HEAD as "tenth-signed". Here,
we are not so lucky. The tag ends up on the same commit as
"eighth-signed-alt", and that's a signed commit, so t7510-signed-commit
will pass, despite `git commit-tree -S` failing.

Make these `git commit-tree` invocations a direct part of the &&-chain,
so that we can rely less on luck and set a better example for future
tests modeled after this one. Fix a 9/10 copy/paste error while at it.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Brandon Richardson <brandon1024.br@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7510-signed-commit.sh

@@ -49,9 +49,13 @@ test_expect_success GPG 'create signed commits' '
 	git tag eighth-signed-alt &&
 
 	# commit.gpgsign is still on but this must not be signed
-	git tag ninth-unsigned $(echo 9 | git commit-tree HEAD^{tree}) &&
+	echo 9 | git commit-tree HEAD^{tree} >oid &&
+	test_line_count = 1 oid &&
+	git tag ninth-unsigned $(cat oid) &&
 	# explicit -S of course must sign.
-	git tag tenth-signed $(echo 9 | git commit-tree -S HEAD^{tree})
+	echo 10 | git commit-tree -S HEAD^{tree} >oid &&
+	test_line_count = 1 oid &&
+	git tag tenth-signed $(cat oid)
 '
 
 test_expect_success GPG 'verify and show signatures' '