changectx: use names api to simplify and extend node lookup

Previously, changectx had to know about each type of name (bookmark, tag, etc.)
to look up. Now, we use repo.namenodes to simplify (and extend) this.

mercurial/context.py

@@ -407,10 +407,14 b' class changectx(basectx):'
                 except (TypeError, LookupError):
                     pass
 
-            if changeid in repo._bookmarks:
-                self._node = repo._bookmarks[changeid]
+            # lookup bookmarks through the name interface
+            try:
+                self._node = repo.names.singlenode(changeid)
                 self._rev = repo.changelog.rev(self._node)
                 return
+            except KeyError:
+                pass
+
             if changeid in repo._tagscache.tags:
                 self._node = repo._tagscache.tags[changeid]
                 self._rev = repo.changelog.rev(self._node)