mercurial: replace ancestor to pure python version for hg 4.1 compatability problems.

this was moved now to debugcommands, but we don't need to run it via commands
as the code is very simple.

vcsserver/hg.py

@@ -674,12 +674,10 b' class HgRemote(object):'
     @reraise_safe_exceptions
     def ancestor(self, wire, revision1, revision2):
         repo = self._factory.repo(wire)
-        baseui = self._factory._create_config(wire['config'])
-        output = io.BytesIO()
-        baseui.write = output.write
-        commands.debugancestor(baseui, repo, revision1, revision2)
-
-        return output.getvalue()
+        changelog = repo.changelog
+        lookup = repo.lookup
+        a = changelog.ancestor(lookup(revision1), lookup(revision2))
+        return hex(a)
 
     @reraise_safe_exceptions
     def push(self, wire, revisions, dest_path, hooks=True,