While not planned, it's also not completely impossible that a tree node
might get additional top-level fields. As the tree iterator is built
with a strict expectation of the top-level fields, this would result in
a parsing error. Future-proof the code by simply skipping unknown
fields.
The TreeNodeIterator decodes nodes while iterating over a tree blob.
This should reduce peak memory usage as now only the serialized tree
blob and a single node have to alive at the same time. Using the
iterator has implications for the error handling however. Now it is
necessary that all loops that iterate through a tree check for errors
before using the node returned by the iterator.
The other change is that it is no longer possible to iterate over a tree
multiple times. Instead it must be loaded a second time. This only
affects the tree rewriting code.
data.TestCreateSnapshot which is used in particular by TestFindUsedBlobs
and TestFindUsedBlobs could generate trees with duplicate file names.
This is invalid and going forward will result in an error.
The new method combines both step into a single wrapper function. Thus
it ensures that both are always called in pairs. As an additional
benefit this slightly reduces the boilerplate to upload blobs.
