hgweb: set sent_headers attr as early as practical

While doing Python 3 porting work, I've seen exceptions happen in
parts of hgweb we normally assume are robust. It won't hurt anything
to set this attribute significantly earlier, so let's do so and save
confusing during the porting process.

Differential Revision: https://phab.mercurial-scm.org/D1085

mercurial/hgweb/server.py

@@ -115,6 +115,7 b' class _httprequesthandler(httpservermod.'
         self.do_POST()
 
     def do_hgweb(self):
+        self.sent_headers = False
         path, query = _splitURI(self.path)
 
         env = {}
@@ -167,7 +168,6 b' class _httprequesthandler(httpservermod.'
 
         self.saved_status = None
         self.saved_headers = []
-        self.sent_headers = False
         self.length = None
         self._chunked = None
         for chunk in self.server.application(env, self._start_response):