hgweb: fetch tipmost unfiltered rev thru the changelog stable

hgweb: fetch tipmost unfiltered rev thru the changelog

This fixes a traceback when tip is filtered (e.g. because it's secret).
See issue3783, for which this is a partial fix.

--- a/mercurial/hgweb/webcommands.py	Fri Jan 25 16:11:16 2013 -0600
+++ b/mercurial/hgweb/webcommands.py	Fri Jan 25 14:50:18 2013 -0600
@@ -188,7 +188,7 @@
         if 'rev' in req.form:
             hi = req.form['rev'][0]
         else:
-            hi = len(web.repo) - 1
+            hi = 'tip'
         try:
             ctx = web.repo[hi]
         except error.RepoError: