make request -> instance conversion support symlink dirs

I think this also clarifies the code, a bit

tests/functional_tests.py

@@ -125,6 +125,20 @@ def test_that_request_to_symlink_redirects_file(client):
     assert response.location == 'http://localhost/foo.txt'
 
 
+def test_that_request_to_symlink_redirects_directory(client):
+    """Test that a request to /foo/ redirects to /what-foo-points-at/."""
+    response = client.get('/symlink-to-subdir/')
+    assert response.status_code == 301
+    assert response.location == 'http://localhost/subdir'
+    # sadly, this location also redirects
+    response = client.get('/subdir')
+    assert response.status_code == 301
+    assert response.location == 'http://localhost/subdir/'
+    # but we do get there
+    response = client.get('/subdir/')
+    assert response.status_code == 200
+
+
 def test_that_dir_request_does_not_redirect(client):
     """Test that a request to /foo/ serves the index page, if foo is a directory."""
     response = client.get('/subdir/')

tests/instance/pages/symlink-to-subdir

@@ -0,0 +1 @@
+subdir

tests/test_pages.py

@@ -146,6 +146,13 @@ def test_request_path_to_instance_resource_path_file_symlink(app):
                 ('pages/foo.txt', 'symlink'))
 
 
+def test_request_path_to_instance_resource_path_dir_symlink(app):
+    """Test that a request for e.g. '/foo' when /foo is a symlink to /bar redirects."""
+    with app.test_request_context():
+        assert (request_path_to_instance_resource_path('symlink-to-subdir/') ==
+                ('pages/subdir', 'symlink'))
+
+
 def test_request_path_to_instance_resource_path_nonexistant_file_errors(app):
     """Test that a request for something not on disk errors."""
     with app.test_request_context():