extractTree has to parse the whole input list in order to generate a tree,
so convert interface to non-streaming.
Some quick memory benchmarks in a repo with 60k files
don't look too bad despite not streaming.
To stream, without building up a whole tree object, one way would
be a new interface:
adjustTree :: MonadIO m :: (TreeItem -> m (Maybe TreeItem)) -> Ref -> Repo -> m Sha
This would only need to buffer tree objects from the current one down
to the root, in order to update trees when a TreeItem is changed.
But, while it supports changing items in the tree, and removing items,
it does not support adding new items, or moving items from one directory to
another.
