formatter: port handling of 'originalnode' to populatemap() hook

This isn't a pure templating business, so let's move out of the templater
module.

Note that this works only in web templates where mapping['node'] is a static
value.

mercurial/formatter.py

@@ -514,6 +514,8 b' class templateresources(templater.resour'
         mapping = {}
         if self._hasctx(newmapping):
             mapping['revcache'] = {}  # per-ctx cache
+        if 'node' in origmapping and 'node' in newmapping:
+            mapping['originalnode'] = origmapping['node']
         return mapping
 
     def _getsome(self, context, mapping, key):

mercurial/templateutil.py

@@ -406,7 +406,6 b' def runmap(context, mapping, data):'
         if isinstance(v, dict):
             lm = context.overlaymap(mapping, v)
             lm['index'] = i
-            lm['originalnode'] = mapping.get('node')
             yield evalrawexp(context, lm, targ)
         else:
             # v is not an iterable of dicts, this happen when 'key'