Add missing submodule imports in git.objects

Since they are listed in __all__. (They are imported either way
because names are imported from them, and this caused them to be
present with the same effect.)

Though they are proably about to be removed along with the
corresponding entries in __all__.

Author: EliahKagan

git/objects/__init__.py

@@ -23,6 +23,7 @@
     "TreeModifier",
 ]
 
+from . import base, blob, commit, submodule, tag, tree
 from .base import IndexObject, Object
 from .blob import Blob
 from .commit import Commit