templatekw: do not directly call .gen

templatekw: do not directly call .gen

--- a/mercurial/templatekw.py	Sat Mar 17 20:52:50 2018 +0900
+++ b/mercurial/templatekw.py	Sun Mar 18 00:11:36 2018 +0900
@@ -613,10 +613,7 @@
 
     # Format the successorssets
     def render(d):
-        t = []
-        for i in d.gen():
-            t.append(i)
-        return "".join(t)
+        return templateutil.stringify(context, mapping, d)
 
     def gen(data):
         yield "; ".join(render(d) for d in data)