pushbookmark: do not attempt to update bookmarks if the push failed (BC)

Before this patch, there was always an attempt to update bookmark even if prior
steps of the push failed. I cannot see a good semantic reason to do so. We
disable this possibility to simplify the push flow with bundle2. Bookmarks will
be included in the bundle and fail with other steps.

mercurial/exchange.py

@@ -613,6 +613,8 b' def _pushobsolete(pushop):'
 
 def _pushbookmark(pushop):
     """Update bookmark position on remote"""
+    if pushop.ret == 0:
+        return
     ui = pushop.ui
     repo = pushop.repo.unfiltered()
     remote = pushop.remote