"Uninitialized" submodules are "unmodified"

Extend the "unmodified" submodule workdir test to include
uninitialized submodules, to prevent reporting submodules as
modified when they're not in the workdir at all.
Merge pull request #2065 from ethomson/submodule_unmodified

"Uninitialized" submodules are "unmodified"