Cannot use `--write-batch` with `--compress-choice=zstd`.

[iago-lito]

It seems that the following works:

$ mkdir a b
$ rsync --compress-choice=zstd a b
skipping directory a

But the following fails:

$ rsync --write-batch --compress-choice=zstd a b
Your --compress-choice value (zstd) was refused by the server.
rsync error: requested action not supported (code 4) at compat.c(449) [Receiver=3.3.0]
skipping directory a
rsync: connection unexpectedly closed (0 bytes received so far) [sender]
rsync error: error in rsync protocol data stream (code 12) at io.c(231) [sender=3.3.0]

It is unclear from the docs why or how to fix it. Possibly this is just not supported yet?

[steadytao]

I opened a small docs PR for this. The current code deliberately forces batch mode through old-style zlib compression negotiatio so the manpage now says --write-batch is not compatible with newer compression choices such as zstd or lz4.