templater: fix list templating bug

High-level use case: printing a list of objects with formatter
when each object in turn contains a list of properties (like
when % template symbol is used in {things % '{thing}'}

Let the top-level list contain one thing with two properties:
objs = [{
   'props': [
     { 'value': 1, 'show': 1 },
     { 'value': 2 }]
}]

(please note that second property does not have 'show' key)

If a templateformatter is used to print this with template
  "{props % '{if(show, value)}'}"
current implementation will print value for both properties,
which is a bug. This happens because in `templater.runmap`
function we only rewrite mapping values with existing new
values for each item. If some mapping value is missing in
the item, it will not be removed.

mercurial/templater.py

@@ -296,9 +296,8 b' def runmap(context, mapping, data):'
     if util.safehasattr(d, 'itermaps'):
         d = d.itermaps()
 
-    lm = mapping.copy()
-
     for i in d:
+        lm = mapping.copy()
         if isinstance(i, dict):
             lm.update(i)
             lm['originalnode'] = mapping.get('node')