httppeer: advertise and support application/mercurial-0.2 Now that servers expose a capability indicating they support application/mercurial-0.2 and compression, clients can key off this to say they support responses that are compressed with various compression formats. After this commit, the HTTP wire protocol client now sends an "X-HgProto-<N>" request header indicating its support for "application/mercurial-0.2" media type and various compression formats. This commit also implements support for handling "application/mercurial-0.2" responses. It simply reads the header compression engine identifier then routes the remainder of the response to the appropriate decompressor. There were some test changes, but only to logging. That points to an obvious gap in our test coverage. This will be addressed in a subsequent commit once server support is in place (it is hard to test without server support).