unpack-trees: initialize fsmonitor_has_run_once in o->result

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>