transports: ssh: replace deprecated function `libssh2_session_startup`

The function `libssh2_session_startup` has been deprecated since libssh2
version 1.2.8 in favor of `libssh2_session_handshake` introduced in the
same version. libssh2 1.2.8 was released in April 2011, so it is already
seven years old. It is available in Debian Wheezy, Ubuntu Trusty and
CentOS 7.4, so the most important and conservative distros already have
it available. As such, it seems safe to just use the new function.

src/transports/ssh.c

@@ -490,7 +490,7 @@ static int _git_ssh_session_create(
 	}
 
 	do {
-		rc = libssh2_session_startup(s, socket->s);
+		rc = libssh2_session_handshake(s, socket->s);
 	} while (LIBSSH2_ERROR_EAGAIN == rc || LIBSSH2_ERROR_TIMEOUT == rc);
 
 	if (rc != LIBSSH2_ERROR_NONE) {