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