If the index contains stat data for a modified file, and the file is
not racily dirty, and there exists an untracked working tree directory
alphabetically after that file, and there are no other changes to the
repo, then git_stash_save would fail. It would confuse the untracked
working tree directory for the modified file, because they have the
same sha: zero.  The wt directory has a sha of zero because it's a
directory, and the file would have a zero sha because we wouldn't read
the file -- we would just know that it doesn't match the index.  To
fix this confusion, we simply check mode as well as SHA.