Merge branch 'nk/diff-files-vs-fsmonitor'

Merge branch 'nk/diff-files-vs-fsmonitor'

"git diff" and other commands that share the same machinery to
compare with working tree files have been taught to take advantage
of the fsmonitor data when available.

* nk/diff-files-vs-fsmonitor:
  p7519-fsmonitor: add a git add benchmark
  p7519-fsmonitor: refactor to avoid code duplication
  perf lint: add make test-lint to perf tests
  t/perf: add fsmonitor perf test for git diff
  t/perf/p7519-fsmonitor.sh: warm cache on first git status
  t/perf/README: elaborate on output format
  fsmonitor: use fsmonitor data in `git diff`