git-gui: Assume untracked directories are Git submodules

git-gui: Assume untracked directories are Git submodules

If `git ls-files --others` returned us the name of a directory then
it is because Git has decided that this directory itself contains a
valid Git repository and its files shouldn't be listed as untracked
for this repository.

In such a case we should label the object as a Git repository and
not just as a directory.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>