Check if submodule exists before referencing

Author: tanaga9

git/diff.py

@@ -283,7 +283,8 @@ def __init__(self, repo, a_rawpath, b_rawpath, a_blob_id, b_blob_id, a_mode,
         if repo and a_rawpath:
             for submodule in repo.submodules:
                 if submodule.path == a_rawpath.decode("utf-8"):
-                    repo = submodule.module()
+                    if submodule.module_exists():
+                        repo = submodule.module()
                     break
 
         if a_blob_id is None or a_blob_id == self.NULL_HEX_SHA: