t7510: use consistent &&-chains in loop

We check multiple commits in a loop. Because we want to
break out of the loop if any single iteration fails, we use
a subshell/exit like:

  (
	for i in $stuff
	do
		do-something $i || exit 1
	done
  )

However, we are inconsistent in our loop body. Some commands
get their own "|| exit 1", and others try to chain to the
next command with "&&", like:

  X &&
  Y || exit 1
  Z || exit 1

This is a little hard to read and follow, because X and Y
are treated differently for no good reason. But much worse,
the second loop follows a similar pattern and gets it wrong.
"Y" is expected to fail, so we use "&& exit 1", giving us:

  X &&
  Y && exit 1
  Z || exit 1

That gets the test for X wrong (we do not exit unless both X
fails and Y unexpectedly succeeds, but we would want to exit
if _either_ is wrong). We can write this clearly and
correctly by consistently using "&&", followed by a single
"|| exit 1", and negating Y with "!" (as we would in a
normal &&-chain). Like:

  X &&
  ! Y &&
  Z || exit 1

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7510-signed-commit.sh

@@ -50,18 +50,18 @@ test_expect_success GPG 'show signatures' '
 		for commit in initial second merge fourth-signed fifth-signed sixth-signed seventh-signed
 		do
 			git show --pretty=short --show-signature $commit >actual &&
-			grep "Good signature from" actual || exit 1
-			! grep "BAD signature from" actual || exit 1
-			echo $commit OK
+			grep "Good signature from" actual &&
+			! grep "BAD signature from" actual &&
+			echo $commit OK || exit 1
 		done
 	) &&
 	(
 		for commit in merge^2 fourth-unsigned sixth-unsigned seventh-unsigned
 		do
 			git show --pretty=short --show-signature $commit >actual &&
-			grep "Good signature from" actual && exit 1
-			! grep "BAD signature from" actual || exit 1
-			echo $commit OK
+			! grep "Good signature from" actual &&
+			! grep "BAD signature from" actual &&
+			echo $commit OK || exit 1
 		done
 	)
 '