lfs: show a friendly message when pushing lfs to a server without lfs enabled Upfront disclaimer: I don't know anything about the wire protocol, and this was pretty much cargo-culted from largefiles, and then clonebundles, since it seems more modern. I was surprised that exchange.push() will ensure all of the proper requirements when exchanging between two local repos, but doesn't care when one is remote. All this new capability marker does is inform the client that the extension is enabled remotely. It may or may not contain commits with external blobs. Open issues: - largefiles uses 'largefiles=serve' for its capability. Someday I hope to be able to push lfs blobs to an `hg serve` instance. That will probably require a distinct capability. Should it change to '=serve' then? Or just add an 'lfs-serve' capability then? - The flip side of this is more complicated. It looks like largefiles adds an 'lheads' command for the client to signal to the server that the extension is loaded. That is then converted to 'heads' and sent through the normal wire protocol plumbing. A client using the 'heads' command directly is kicked out with a message indicating that the largefiles extension must be loaded. We could do similar with 'lfsheads', but then a repo with both largefiles and lfs blobs can't be pushed over the wire. Hopefully somebody with more wire protocol experience can think of something else. I see 'x-hgarg-1' on some commands in the tests, but not on heads, and didn't dig any further.