pull: skip pulling remote bookmarks with bundle1 if a value already exist

For efficiency and consistency purpose, remote bookmarks, retrieved at the time
the pull command code is doing lookup, will be reused during the core pull
operation.

A first step toward this is to setup the logic avoiding pulling the data again
during the discovery phase if some have already been provided.

mercurial/exchange.py

@@ -949,6 +949,8 b' def _pullbookmarkbundle1(pullop):'
 
     If not using bundle2, we have to fetch bookmarks before changeset
     discovery to reduce the chance and impact of race conditions."""
+    if pullop.remotebookmarks is not None:
+        return
     if not _canusebundle2(pullop): # all bundle2 server now support listkeys
         pullop.remotebookmarks = pullop.remote.listkeys('bookmarks')