Support username expansion in SSHGitClient

Use same expansion present in TCPGitClient

Signed-off-by: Jelmer Vernooij <jelmer@samba.org>
Add test_connect for SSHGitClient

Mock SSHVendor.connect_ssh and verify arguments

Signed-off-by: Jelmer Vernooij <jelmer@samba.org>
Update NEWS for SSH client ~ expansion patch.