Feature request: support 'scp -3'-like feature

[wxup]

Sometimes we need to sync files from host1 and host3, but host1 and host3 cannot connect directly, but can be reached from host2. And host2 is like a gate server. And due to some special network policy, you can only ssh to host1 from host2, but not vice versa. And there is not ssh-server running in host2. Likewise, you can only ssh into host3 from host2 but not vice versa.

In this case, we can use scp -3 to cp files between host1 and host3 when we are on host2. But scp cannot cp files incrementally. So we hope rsync can support a param like -3 to achieve this effect. Thanks!