Merge branch 'us/unpack-trees-fsmonitor'

Users of oneway_merge() (like "reset --hard") learned to take
advantage of fsmonitor to avoid unnecessary lstat(2) calls.

* us/unpack-trees-fsmonitor:
  unpack-trees: skip stat on fsmonitor-valid files

diff --cc t/t7519-status-fsmonitor.sh
Simple merge

diff --cc unpack-trees.c
Simple merge