remote: usability: "diff" with no args makes nonsensical suggestion (--latest)

[edsantiago]

$ bin/podman-remote diff
Error: "podman-remote diff" requires a name, id, or the "--latest" flag

Solution: fix this to check for IsRemote() https://github.com/containers/podman/blob/b8d95a5893572b37c8257407e964ad06ba87ade6/cmd/podman/diff/diff.go#L75-L77