t7528: work around ETOOMANY in OpenSSH 10.1 and newer

In t7528 we spawn an SSH agent to verify that we can sign a commit via
it. This test has started to fail on some machines:

    +++ ssh-agent
    unix_listener_tmp: path "/home/pks/Development/git/build/test-output/trash directory.t7528-signed-commit-ssh/.ssh/agent/s.UTulegefEg.agent.UrPHumMXPq" too long for Unix domain socket
    main: Couldn't prepare agent socket

As it turns out this is caused by a change in OpenSSH 10.1 [1]:

 * ssh-agent(1), sshd(8): move agent listener sockets from /tmp to
   under ~/.ssh/agent for both ssh-agent(1) and forwarded sockets
   in sshd(8).

Instead of creating the socket in "/tmp", OpenSSH now creates the socket
in our home directory. And as the home directory gets modified to be
located in our test output directory we end up with paths that are
somewhat long. But Linux has a rather short limit of 108 characters for
socket paths, and other systems have even lower limits, so it is very
easy now to exceed the limit and run into the above error.

Work around the issue by using `ssh-agent -T`, which instructs it to
use the old behaviour and create the socket in "/tmp" again. This switch
has only been introduced with 10.1 though, so for older versions we have
to fall back to not using it. That's fine though, as older versions know
to put the socket into "/tmp" already.

An alternative approach would be to abbreviate the socket name itself so
that we create it as e.g. "sshsock" in the trash directory. But taking
the above example we'd still end up with a path that is 91 characters
long. So we wouldn't really have a lot of headroom, and it is quite
likely that some developers would see the issue on their machines.

[1]: https://www.openssh.com/txt/release-10.1

Reported-by: Xi Ruoyao <[email protected]>
Suggested-by: brian m. carlson <[email protected]>
Helped-by: Jeff King <[email protected]>
Helped-by: Lauri Tirkkonen <[email protected]>
Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

t/t7528-signed-commit-ssh.sh

@@ -82,7 +82,7 @@ test_expect_success GPGSSH 'create signed commits' '
 test_expect_success GPGSSH 'sign commits using literal public keys with ssh-agent' '
 	test_when_finished "test_unconfig commit.gpgsign" &&
 	test_config gpg.format ssh &&
-	eval $(ssh-agent) &&
+	eval $(ssh-agent -T || ssh-agent) &&
 	test_when_finished "kill ${SSH_AGENT_PID}" &&
 	test_when_finished "test_unconfig user.signingkey" &&
 	mkdir tmpdir &&