Traverse the entries in a tree and its subtrees in post or pre order
The entries will be traversed in the specified order,
children subtrees will be automatically loaded as required,
and the callback
will be called once per entry with
the current (relative) root for the entry and the entry
data itself.
If the callback returns a negative value, the passed entry will be skiped on the traversal.
Traversal mode (pre or post-order)
Opaque pointer to be passed on each callback
GIT_SUCCESS or an error code