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