This makes sure that an error code returned by the callback function
of `git_tree_walk` will stop the iteration and get propagated back
to the caller verbatim.

Also, this adds a minor helper function `git_tree_entry_byoid` that
searches a `git_tree` for an entry with the given OID.  This isn't
a fast function, but it's easier than writing the loop yourself as
an external user of the library.