httppeer: basic implementation of capabilities interface This is a bit crude. The capabilities mechanism for version 2 of the wire protocol is a bit different from version 1. And code in core is relying on strings passed to capable() matching strings advertised by the "capabilities" wire protocol command. I may refactor the internal checking mechanism to be a bit more abstract or based on interfaces. Time will tell... Differential Revision: https://phab.mercurial-scm.org/D3256