test: use proper subclassing in `test-issue2137.t`.

To use changelog filtering on the repository, we plan to use "proxy" object that
perfectly mock a repository but with a filtered changelog.

Altering the `repo.commit` function using `extensions.wrapfunction` will prevent
the logic to propagate to the proxy class by the mean of inheritance.

We changes the extension to use subclassing as expectable.

tests/test-issue2137.t

@@ -12,15 +12,15 b' 1 and 2 are preconditions for the bug; 3'
   > from mercurial import extensions, node, revlog
   > 
   > def reposetup(ui, repo):
-  >     def wrapcommit(orig, *args, **kwargs):
-  >         result = orig(*args, **kwargs)
+  >     class wraprepo(repo.__class__):
+  >         def commit(self, *args, **kwargs):
+  >             result = super(wraprepo, self).commit(*args, **kwargs)
   >         tip1 = node.short(repo.changelog.tip())
   >         tip2 = node.short(repo.lookup(tip1))
   >         assert tip1 == tip2
   >         ui.write('new tip: %s\n' % tip1)
   >         return result
-  > 
-  >     extensions.wrapfunction(repo, 'commit', wrapcommit)
+  >     repo.__class__ = wraprepo
   > 
   > def extsetup(ui):
   >     revlog._maxinline = 8             # split out 00changelog.d early