bookmarks: switch from repo.changectx('.') to repo['.']

The two forms are synonymous and the new form is by far the more
common form.

Differential Revision: https://phab.mercurial-scm.org/D3033

mercurial/bookmarks.py

@@ -204,7 +204,7 b' class bmstore(dict):'
 
         If divergent bookmark are to be deleted, they will be returned as list.
         """
-        cur = self._repo.changectx('.').node()
+        cur = self._repo['.'].node()
         if mark in self and not force:
             if target:
                 if self[mark] == target and target == cur:
@@ -818,7 +818,7 b' def addbookmarks(repo, tr, names, rev=No'
     Raises an abort error if old is not in the bookmark store.
     """
     marks = repo._bookmarks
-    cur = repo.changectx('.').node()
+    cur = repo['.'].node()
     newact = None
     changes = []
     hiddenrev = None