Convert some leftover changeish mentions to ref

Noticed there were a few vestigal references to Changeish that didn't get
converted, including the ChangeishFilter. Convert them to Ref.

In model, there were two places where a method started with changeish =
self.change. Those were removed and self.change was used in the method
because ref = self.change led to the method having two important
variables "ref" and "ret" which seems like a recipe for confusion. Of
course, that led to one line being too long for pep8, so it was
rewritten to use getattr instead of hasattr + and.

Change-Id: I420b8ec7c7eb32574dfe2de467fc2bbd0f96e9f1
diff --git a/doc/source/developer/datamodel.rst b/doc/source/developer/datamodel.rst
index acb8612..c4ff4a0 100644
--- a/doc/source/developer/datamodel.rst
+++ b/doc/source/developer/datamodel.rst
@@ -8,7 +8,7 @@
 
 Pipelines have a configured
 :py:class:`~zuul.manager.PipelineManager` which controlls how
-the :py:class:`Change <zuul.model.Changeish>` objects are enqueued and
+the :py:class:`Ref <zuul.model.Ref>` objects are enqueued and
 processed.
 
 There are currently two,
@@ -35,7 +35,7 @@
 .. autoclass:: zuul.model.Build
 
 The :py:class:`~zuul.manager.base.PipelineManager` enqueues each
-:py:class:`Change <zuul.model.Changeish>` into the
+:py:class:`Ref <zuul.model.Ref>` into the
 :py:class:`~zuul.model.ChangeQueue` in a :py:class:`~zuul.model.QueueItem`.
 
 .. autoclass:: zuul.model.QueueItem