Only reverse adjust the changes in the commit, which means that adjustments
do not need to be generally cleanly reversable.
For example, an adjustment can unlock all locked files, but does not need
to worry about files that were originally unlocked when reversing, because
it will only ever be run on files that have been changed. So, it's ok
if it locks all files when reversed, or even leaves all files as-is when
reversed.
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.
