[MERGE] Fix PreviewTree.annotate

[Aaron Bentley]

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John Arbash Meinel wrote:
> John Arbash Meinel has voted tweak.
> Status is now: Conditionally approved
> Comment:
> Just layering a annotate.reannotate() on top of extracting the text IFF
> the text should have changed. Seems good.
> 
> Though you don't test annotate_iter() when there are no changes. I would
> like you to, just to exercise more code paths.

That is already exercised by existing tests.  Every public method on
PreviewTree is tested.  However, they are tested only for the case where
there are no changes.

These are the tests of no-change functionality:
tree_implementations.test_annotate_iter.TestAnnotate.test_annotate_simple(PreviewTree)

tree_implementations.test_annotate_iter.TestAnnotate.test_annotate_with_ghost(PreviewTree)

tree_implementations.test_tree.TestAnnotate.test_annotate(PreviewTree)

> And maybe also if you change something which *isn't* a content change.
> Like maybe renaming the file?

I'm happy to do that.

Aaron
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