How to disable check for renames?

Hi,

I've tried to use the following to disable git diff renamed files search:
```
repo.index.diff('origin/HEAD', no_renames=True)
```

This did not work, and after some digging, i've found that GitPython hardcodes the `-M` flag:
https://github.com/gitpython-developers/GitPython/blob/master/git/diff.py#L106

Why is this flag hardcoded and is there a way around it?

Thanks
