Merge branch 'nk/diff-index-fsmonitor'

"git diff-index" codepath has been taught to trust fsmonitor status
to reduce number of lstat() calls.

* nk/diff-index-fsmonitor:
  fsmonitor: add perf test for git diff HEAD
  fsmonitor: add assertion that fsmonitor is valid to check_removed
  fsmonitor: skip lstat deletion check during git diff-index