tea previously set GIT_SSH_COMMAND=ssh for every SSH operation, even when
it had no identity or passphrase to inject. Let git fall back to its own
SSH command in that case so core.sshCommand, GIT_SSH_COMMAND, and the
normal OpenSSH known_hosts handling are respected.
Add regression coverage for ed25519 key parsing and CLI SSH auth setup.
Signed-off-by: Lunny Xiao <xiaolunwen@gmail.com>
