templatekw: have {manifest} use ctx.manifestnode() for consistency

changeset_printer was updated at f57640bf10d4 to not access changeset by
index.

mercurial/templatekw.py

@@ -333,9 +333,9 b' def showlatesttagdistance(repo, ctx, tem'
 
 def showmanifest(**args):
     repo, ctx, templ = args['repo'], args['ctx'], args['templ']
+    mnode = ctx.manifestnode()
     args = args.copy()
-    args.update({'rev': repo.manifest.rev(ctx.changeset()[0]),
-                 'node': hex(ctx.changeset()[0])})
+    args.update({'rev': repo.manifest.rev(mnode), 'node': hex(mnode)})
     return templ('manifest', **args)
 
 def shownode(repo, ctx, templ, **args):