ssh signing: make sign/amend test more resilient

The test `amending already signed commit` is using git checkout to
select a specific commit to amend. In case an earlier test fails and
leaves behind a dirty index/worktree this test would fail as well.
Using `checkout -f` will avoid interference by most other tests.

Signed-off-by: Fabian Stelzer <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: FStelzer

t/t7510-signed-commit.sh

@@ -228,7 +228,7 @@ test_expect_success GPG 'detect fudged signature with NUL' '
 '
 
 test_expect_success GPG 'amending already signed commit' '
-	git checkout fourth-signed^0 &&
+	git checkout -f fourth-signed^0 &&
 	git commit --amend -S --no-edit &&
 	git verify-commit HEAD &&
 	git show -s --show-signature HEAD >actual &&

t/t7528-signed-commit-ssh.sh

@@ -239,7 +239,7 @@ test_expect_success GPGSSH 'amending already signed commit' '
 	test_config gpg.format ssh &&
 	test_config user.signingkey "${GPGSSH_KEY_PRIMARY}" &&
 	test_config gpg.ssh.allowedSignersFile "${GPGSSH_ALLOWED_SIGNERS}" &&
-	git checkout fourth-signed^0 &&
+	git checkout -f fourth-signed^0 &&
 	git commit --amend -S --no-edit &&
 	git verify-commit HEAD &&
 	git show -s --show-signature HEAD >actual &&