Ignore git index extension data when writing index
Ignore git index extension data when writing index

This removes some cached tree data that should be invalidated
by our changes to the index (assuming we've added/updated things).
Otherwise, `git commit` will not commit changes, even though
they are visible to `git status`.

See https://github.com/gitpython-developers/GitPython/issues/1185

tymorrow/assigner,redkyn/assigner