push: introduce a discovery step for obsmarker

The discovery step is still not doing anything smart. But this will allow
extension to wrap it.

mercurial/exchange.py

@@ -82,7 +82,7 b' class pushoperation(object):'
         # phases changes that must be pushed if changeset push fails
         self.fallbackoutdatedphases = None
         # outgoing obsmarkers
-        self.outobsmarkers = repo.obsstore
+        self.outobsmarkers = set()
 
     @util.propertycache
     def futureheads(self):
@@ -278,6 +278,10 b' def _pushdiscoveryphase(pushop):'
     pushop.outdatedphases = future
     pushop.fallbackoutdatedphases = fallback
 
+@pushdiscovery('obsmarker')
+def _pushdiscoveryobsmarkers(pushop):
+    pushop.outobsmarkers = pushop.repo.obsstore
+
 def _pushcheckoutgoing(pushop):
     outgoing = pushop.outgoing
     unfi = pushop.repo.unfiltered()