Apparently, git 1.7.9 supports git-files too, lets assume it's starting at 1.7.0

Byron · Byron · commit e2feb62c17ac · 2015-02-20T18:11:13.000+01:00
diff --git a/git/objects/submodule/base.py b/git/objects/submodule/base.py
@@ -136,8 +136,7 @@ def _get_intermediate_items(self, item):
 
     @classmethod
     def _need_gitfile_submodules(cls, git):
-        # on debian wheezy, it seems to already support git files at this version. Maybe even earler, who knows
-        return git.version_info[:3] >= (1, 7, 10)
+        return git.version_info[:3] >= (1, 7, 0)
 
     def __eq__(self, other):
         """Compare with another submodule"""
