hgweb: separate protocol calls from interface calls (issue996)

The protocol functions are already pretty careful about not raising
exceptions to the caller, and have their own error handling. We can formalize
this a little bit to make it clearer (before, the exception handlers for
a limited number of exceptions coming from the interface bits would blow up
because some variables aren't instantiated for the protocol calls).

mercurial/hgweb/hgweb_mod.py

@@ -199,15 +199,18 b' class hgweb(object):'
                         req.form['node'] = [fn[:-len(ext)]]
                         req.form['type'] = [type_]
 
-        # actually process the request
-
-        try:
+        # process this if it's a protocol request
 
             cmd = req.form.get('cmd', [''])[0]
             if cmd in protocol.__all__:
                 method = getattr(protocol, cmd)
                 method(self, req)
-            else:
+            return
+
+        # process the web interface request
+
+        try:
+
                 tmpl = self.templater(req)
                 ctype = tmpl('mimetype', encoding=self.encoding)
                 ctype = templater.stringify(ctype)