bookmarks: use isrevsymbol() for detecting collision with existing symbol

Differential Revision: https://phab.mercurial-scm.org/D3167

mercurial/bookmarks.py

@@ -238,7 +238,7 b' class bmstore(dict):'
                 _("a bookmark cannot have the name of an existing branch"))
         if len(mark) > 3 and not force:
             try:
-                shadowhash = (mark in self._repo)
+                shadowhash = scmutil.isrevsymbol(self._repo, mark)
             except error.LookupError:  # ambiguous identifier
                 shadowhash = False
             if shadowhash: