This feature had been intended for cases where a module kde/foo was a
git repository while also being a base for other git repositories
kde/foo/bar, kde/foo/baz, etc. For instance, kde/kdelibs is a git
repository that also held sub-children like kde/kdelibs/kactivities.

However this is fairly rare in KDE repositories now, and this feature in
its easiest-to-implement form has always had to navigate around several
corner cases, so it's better just to explicitly specify dependencies.