unpack-trees: initialize fsmonitor_has_run_once in o->result

Initialize `o->result.fsmonitor_has_run_once` based upon value in `o->src_index->fsmonitor_has_run_once` to prevent a second fsmonitor query during the tree traversal and possibly getting a skewed view of the working directory. The checkout code has already talked to the fsmonitor and the traversal is updating the index as it traverses, so there is no need to query the fsmonitor. Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>