push: use stepsdone for obsmarkers push

We do not have infrastructure to include obsolescence markers in the bundle2
push from core. But extensions may so we make sure it would not be sent twice.

mercurial/exchange.py

@@ -590,9 +590,12 b' def _localphasemove(pushop, nodes, phase'
 
 def _pushobsolete(pushop):
     """utility function to push obsolete markers to a remote"""
+    if 'obsmarkers' in pushop.stepsdone:
+        return
     pushop.ui.debug('try to push obsolete markers to remote\n')
     repo = pushop.repo
     remote = pushop.remote
+    pushop.stepsdone.add('obsmarkers')
     if (obsolete._enabled and repo.obsstore and
         'obsolete' in remote.listkeys('namespaces')):
         rslts = []