Add a new entry to a tree, returning that new entry. The only difference with this call is that it does not sort tree afterwards, this requirement is left to the caller.
This will mark the tree as modified; the new entry will be written back to disk on the next git_object_write()
Entry object which will be created
Tree object to store the entry \iparam id OID for the tree entry
Filename for the tree entry
UNIX file attributes for the entry
0 on success; otherwise error code