hgweb: wrap {whyunstable} with mappinggenerator

This is also a generator of mappings, which needs a wrapper.

mercurial/hgweb/webutil.py

@@ -392,7 +392,7 b' def succsandmarkers(context, mapping):'
 # teach templater succsandmarkers is switched to (context, mapping) API
 succsandmarkers._requires = {'repo', 'ctx'}
 
-def whyunstable(context, mapping):
+def _whyunstablegen(context, mapping):
     repo = context.resource(mapping, 'repo')
     ctx = context.resource(mapping, 'ctx')
 
@@ -402,6 +402,9 b' def whyunstable(context, mapping):'
             entry['divergentnodes'] = _siblings(entry['divergentnodes'])
         yield entry
 
+def whyunstable(context, mapping):
+    return templateutil.mappinggenerator(_whyunstablegen, args=(mapping,))
+
 whyunstable._requires = {'repo', 'ctx'}
 
 def commonentry(repo, ctx):