repoview: wrap changelog class when filtering

The class doesn't yet do anything. I'll move the filter-aware
overrides from the changelog class over one by one to this class in
coming patches. That will leave the changelog class simpler and will
centralize more of the filtering logic to repoview. I could not
measure any performance difference.

Differential Revision: https://phab.mercurial-scm.org/D7236

mercurial/repoview.py

@@ -221,6 +221,12 b' def filterrevs(repo, filtername, visibil'
 def wrapchangelog(unfichangelog, filteredrevs):
     cl = copy.copy(unfichangelog)
     cl.filteredrevs = filteredrevs
+
+    class filteredchangelog(cl.__class__):
+        pass
+
+    cl.__class__ = filteredchangelog
+
     return cl