Merge branch 'tk/t7063-chmtime-dirs-too'

Teach "test-chmtime" to work on a directory and use it to avoid
having to wait for a second in a few places in tests.

* tk/t7063-chmtime-dirs-too:
  t7063: mtime-mangling instead of delays in untracked cache testing
  t/helper/test-chmtime: update mingw to support chmtime on directories