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.

mercurial/hgweb/webcommands.py

@@ -188,7 +188,7 b' def changelog(web, req, tmpl, shortlog=F'
         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: