hgweb: open server logs in binary mode

This is consistent with when we're logging to stdout, so we don't have
to do something annoyingly complicated in the logging infrastructure.

Differential Revision: https://phab.mercurial-scm.org/D2294

mercurial/hgweb/server.py

@@ -273,7 +273,7 b' except ImportError:'
 
 def openlog(opt, default):
     if opt and opt != '-':
-        return open(opt, 'a')
+        return open(opt, 'ab')
     return default
 
 class MercurialHTTPServer(_mixin, httpservermod.httpserver, object):