git_filter: dup the filter name

Function `git_filter_register` takes an argument 'name' by which the filter can be referenced. It does not make a copy of name to register it with internal data structure.
If the caller free's the memory pointed by 'name' or overwrites its contents, unregistering filter using `git_filter_unregister` with the same filter name will fail.
Sorry for the late reply to this, would you mind testing for OOM here?