hgweb: wrap {nextentry} of graph with mappinglist

It's a 0/1-length list of a mapping.

mercurial/hgweb/webcommands.py

@@ -1365,7 +1365,7 b' def graph(web):'
         rows=rows,
         bg_height=bg_height,
         changesets=count,
-        nextentry=nextentry,
+        nextentry=templateutil.mappinglist(nextentry),
         jsdata=lambda **x: jsdata(),
         nodes=lambda **x: nodes(),
         node=ctx.hex(),