hgweb: override the charset attribute in the http request

The default http request character set is UTF-8. If the message is not encoded
in UTF-8, such as big5, it cannot be shown correctly. The 'charset' is
overridden by the root document's, such that the user can select the proper
encoding in the browser.

mercurial/templates/static/mercurial.js

@@ -327,6 +327,7 b' function makeRequest(url, method, onstar'
     };
 
     xfr.open(method, url);
+    xfr.overrideMimeType("text/xhtml; charset=" + document.characterSet.toLowerCase());
     xfr.send();
     onstart();
     return xfr;