Merge branch 'pb/stash-refuse-to-kill'

Merge branch 'pb/stash-refuse-to-kill'

"git stash save" is not just about "saving" the local changes, but
also is to restore the working tree state to that of HEAD. If you
changed a non-directory into a directory in the local change, you
may have untracked files in that directory, which have to be killed
while doing so, unless you run it with --include-untracked.  Teach
the command to detect and error out before spreading the damage.

This needed a small fix to "ls-files --killed".

* pb/stash-refuse-to-kill:
  git stash: avoid data loss when "git stash save" kills a directory
  treat_directory(): do not declare submodules to be untracked