porcelain: honor Git SSH command env vars

[aouxwoux]

Summary

• restore porcelain-level GIT_SSH_COMMAND / GIT_SSH handling for remote transports
• keep explicit ssh_command= precedence over environment defaults
• add regression coverage for SSH command env precedence and porcelain transport forwarding

Fixes #2209

Verification

• python -m unittest tests.cli.test_cli.SshCommandFromEnvTest tests.porcelain.TransportKwargsTests tests.test_client.SSHGitClientTests
• python -m ruff check dulwich/porcelain/__init__.py tests/porcelain/__init__.py
• git diff --check

Also attempted python -m unittest tests.porcelain on Windows; it reached unrelated symlink tests that require symlink privileges and failed with WinError 1314.

[jelmer]

I realize this is still marked as draft, but please note that GIT_SSH_COMMAND is now in dulwich.cli; we shouldn't support it in two places. If you think it should be elsewhere, let's have that discussion first.

[aouxwoux]

Thanks, agreed. I pushed a follow-up that removes the porcelain-level GIT_SSH_COMMAND / GIT_SSH lookup and the tests for it, so this no longer adds a second place that supports those env vars.

For #2209, the remaining question is policy: the repro is via porcelain.clone(), while env handling currently lives in dulwich.cli. I'll leave this as draft unless you'd like to discuss moving that behavior or documenting explicit ssh_command= for porcelain callers.