debugcommands: perform handshake when obtaining httpv2 peer If we obtain an httpv2peer directly, the instance doesn't have an API descriptor and therefore doesn't know about the remote's commands, feature support, etc. This doesn't matter now. But when we implement the peer so it consults the API descriptor as part of sending commands, it will. So we change the logic for obtaining an http version 2 peer to go through makepeer() so the peer will perform the handshake and pass the API descriptor to the httpv2peer instance. Tests changed because we now perform a ?cmd=capabilities when obtaining version 2 peers. The Content-Length header is globbed because compression info will lack zstandard for pure builds. Differential Revision: https://phab.mercurial-scm.org/D3296