url: add distribution and version to user-agent request header (BC) As a server operator, I've always wanted to know what Mercurial version clients are running so I can track version adoption and make informed decisions about which versions of Mercurial to support in extensions. Unfortunately, there is no easy way to discern this today: the best you can do is look for high-level feature usage (e.g. bundle2) or sniff capabilities from bundle2 commands. And these things aren't changed frequently enough to tell you anything that interesting. Nearly every piece of software talking HTTP sends its version in the user agent. This includes web browsers, curl, and even Git. This patch adds the distribution name and version to the user-agent HTTP request header. We choose "Mercurial" for the distribution name because that seems appropriate. The version string comes from __version__. The value is inside parenthesis for a few reasons: * The version *may* contain spaces * Alternate forms like "Mercurial/<version>" imply structure and since the user agent should not be used by servers for protocol or feature negotiation/detection, we don't want to even give the illusion that the value should be parsed. A free form field is the most hostile to parsing. Flagging the patch as BC so it shows up in release notes. This change should be backwards compatible. But I wouldn't be surprised if a server somewhere is filtering on the exact old user agent string. So I want to make noise about this change.