http: bump the pretend git version in the User-Agent

We want to keep the git UA in order for services to recognise that we're
a Git client and not a browser. But in order to stop dumb HTTP some
services have blocked UAs that claim to be pre-1.6.6 git.

Thread these needles by using the "git/2.0" prefix which is still close
enough to git's yet distinct enough that you can tell it's us.

src/transports/http.c

@@ -208,7 +208,7 @@ static int gen_request(
 
 	git_buf_printf(buf, "%s %s%s HTTP/1.1\r\n", s->verb, path, s->service_url);
 
-	git_buf_printf(buf, "User-Agent: git/1.0 (%s)\r\n", user_agent());
+	git_buf_printf(buf, "User-Agent: git/2.0 (%s)\r\n", user_agent());
 	git_buf_printf(buf, "Host: %s\r\n", t->connection_data.host);
 
 	if (s->chunked || content_length > 0) {