httpclient: clear the read_buf on new requests

The httpclient implementation keeps a `read_buf` that holds the data
in the body of the response after the headers have been written.  We
store that data for subsequent calls to `git_http_client_read_body`.  If
we want to stop reading body data and send another request, we need to
clear that cached data.

Clear the cached body data on new requests, just like we read any
outstanding data from the socket.

src/transports/httpclient.c

@@ -1195,7 +1195,7 @@ static void complete_response_body(git_http_client *client)
 	/* If we're not keeping alive, don't bother. */
 	if (!client->keepalive) {
 		client->connected = 0;
-		return;
+		goto done;
 	}
 
 	parser_context.client = client;
@@ -1209,6 +1209,9 @@ static void complete_response_body(git_http_client *client)
 		git_error_clear();
 		client->connected = 0;
 	}
+
+done:
+	git_buf_clear(&client->read_buf);
 }
 
 int git_http_client_send_request(