[stagingView] Enhance IndexDiffData to support submodules

By enhancing IndexDiffData to support submodules like we did earlier for
symlinks (see ceee3050) we can avoid a SubmoduleWalk otherwise needed to
decorate modified submodules in the staging view.

Bug: 439674
Change-Id: I366a13f81f82acd7ddab084571c44d6fb51bbf2d
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>