hook: also use pprint on lists for stable output on py2/3

Differential Revision: https://phab.mercurial-scm.org/D3361

mercurial/hook.py

@@ -137,7 +137,7 b' def _exthook(ui, repo, htype, name, cmd,'
     for k, v in args.iteritems():
         if callable(v):
             v = v()
-        if isinstance(v, dict):
+        if isinstance(v, (dict, list)):
             v = stringutil.pprint(v, bprefix=False)
         env['HG_' + k.upper()] = v