Merge branch 'tk/empty-untracked-cache'

Merge branch 'tk/empty-untracked-cache'

The untracked cache newly computed weren't written back to the
on-disk index file when there is no other change to the index,
which has been corrected.

* tk/empty-untracked-cache:
  untracked-cache: write index when populating empty untracked cache
  t7519: populate untracked cache before test
  t7519: avoid file to index mtime race for untracked cache