tree-diff: clear parent array in path_appendnew()

tree-diff: clear parent array in path_appendnew()

All of the other functions which allocate a combine_diff_path struct
zero out the parent array, but this code path does not. There's no bug,
since our caller will fill in most of the fields. But leaving the unused
fields (like combine_diff_parent.path) uninitialized makes working with
the struct more error-prone than it needs to be.

Let's just zero the parent field to be consistent with the
combine_diff_path_new() allocator.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>