t7510: stop referring to master in later tests

Our setup creates a sequence of commits, each with its own
tag. However, we sometimes refer to "seventh-signed" as
"master". This works, since it is at the tip of the created
branch, but is brittle if new tests need to add more
commits. Let's use its tag name to be unambiguous.

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

Author: peff

t/t7510-signed-commit.sh

@@ -47,7 +47,7 @@ test_expect_success GPG 'create signed commits' '
 
 test_expect_success GPG 'show signatures' '
 	(
-		for commit in initial second merge fourth-signed fifth-signed sixth-signed master
+		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
@@ -67,7 +67,7 @@ test_expect_success GPG 'show signatures' '
 '
 
 test_expect_success GPG 'detect fudged signature' '
-	git cat-file commit master >raw &&
+	git cat-file commit seventh-signed >raw &&
 
 	sed -e "s/seventh/7th forged/" raw >forged1 &&
 	git hash-object -w -t commit forged1 >forged1.commit &&
@@ -77,7 +77,7 @@ test_expect_success GPG 'detect fudged signature' '
 '
 
 test_expect_success GPG 'detect fudged signature with NUL' '
-	git cat-file commit master >raw &&
+	git cat-file commit seventh-signed >raw &&
 	cat raw >forged2 &&
 	echo Qwik | tr "Q" "\000" >>forged2 &&
 	git hash-object -w -t commit forged2 >forged2.commit &&