httprepo: long arguments support (issue2126) Send the command arguments in the HTTP headers. The command is still part of the URL. If the server does not have the 'httpheader' capability, the client will send the command arguments in the URL as it did previously. Web servers typically allow more data to be placed within the headers than in the URL, so this approach will: - Avoid HTTP errors due to using a URL that is too large. - Allow Mercurial to implement a more efficient wire protocol. An alternate approach is to send the arguments as part of the request body. This approach has been rejected because it requires the use of POST requests, so it would break any existing configuration that relies on the request type for authentication or caching. Extensibility: - The header size is provided by the server, which makes it possible to introduce an hgrc setting for it. - The client ignores the capability value after the first comma, which allows more information to be included in the future.