allow slash in root->tree redirect

before  #6117, only `/base_url/` worked,
after  #6117, only `/base_url worked,
after this PR, both should work.

IPython/html/tree/handlers.py

@@ -1,84 +1,84 b''
 """Tornado handlers for the tree view."""
 
 # Copyright (c) IPython Development Team.
 # Distributed under the terms of the Modified BSD License.
 
 from tornado import web
 from ..base.handlers import IPythonHandler, notebook_path_regex, path_regex
 from ..utils import url_path_join, url_escape
 
 
 class TreeHandler(IPythonHandler):
     """Render the tree view, listing notebooks, clusters, etc."""
 
     def generate_breadcrumbs(self, path):
         breadcrumbs = [(url_escape(url_path_join(self.base_url, 'tree')), '')]
         comps = path.split('/')
         ncomps = len(comps)
         for i in range(ncomps):
             if comps[i]:
                 link = url_escape(url_path_join(self.base_url, 'tree', *comps[0:i+1]))
                 breadcrumbs.append((link, comps[i]))
         return breadcrumbs
 
     def generate_page_title(self, path):
         comps = path.split('/')
         if len(comps) > 3:
             for i in range(len(comps)-2):
                 comps.pop(0)
         page_title = url_path_join(*comps)
         if page_title:
             return page_title+'/'
         else:
             return 'Home'
 
     @web.authenticated
     def get(self, path='', name=None):
         path = path.strip('/')
         cm = self.contents_manager
         if name is not None:
             # is a notebook, redirect to notebook handler
             url = url_escape(url_path_join(
                 self.base_url, 'notebooks', path, name
             ))
             self.log.debug("Redirecting %s to %s", self.request.path, url)
             self.redirect(url)
         else:
             if not cm.path_exists(path=path):
                 # Directory is hidden or does not exist.
                 raise web.HTTPError(404)
             elif cm.is_hidden(path):
                 self.log.info("Refusing to serve hidden directory, via 404 Error")
                 raise web.HTTPError(404)
             breadcrumbs = self.generate_breadcrumbs(path)
             page_title = self.generate_page_title(path)
             self.write(self.render_template('tree.html',
                 page_title=page_title,
                 notebook_path=path,
                 breadcrumbs=breadcrumbs
             ))
 
 
 class TreeRedirectHandler(IPythonHandler):
     """Redirect a request to the corresponding tree URL"""
 
     @web.authenticated
     def get(self, path=''):
         url = url_escape(url_path_join(
             self.base_url, 'tree', path.strip('/')
         ))
         self.log.debug("Redirecting %s to %s", self.request.path, url)
         self.redirect(url)
 
 
 #-----------------------------------------------------------------------------
 # URL to handler mappings
 #-----------------------------------------------------------------------------
 
 
 default_handlers = [
     (r"/tree%s" % notebook_path_regex, TreeHandler),
     (r"/tree%s" % path_regex, TreeHandler),
     (r"/tree", TreeHandler),
-    (r"", TreeRedirectHandler),
+    (r"/?", TreeRedirectHandler),
     ]