narrow: add narrow and ellipses as server capabilities Right now we don't have a way to differentiate between whether a server can serve ellipsis or not. The way we check whether a server is narrow enabled is by checking bundle2 capability which does not scale outside of bundle2 world. The goal is to use have wireprotocol commands just like remotefilelog for widening the narrow clone, atleast in non-ellipses cases. Having a server capability will help there as we can't rely on bundle2 capability there. Also having a server capability is neat than having a bundle2 capability. There are lot of things we can optimize locally on the client side by knowing before hand that whether the server supports ellipses or not. This will also help us in making sure that a client ellipses repo does not communicate with a server repo without ellipses. Differential Revision: https://phab.mercurial-scm.org/D4521