Add git_index_forget_file

[Cykelero]

This adds git_index_forget_file:

Forget the file associated with this index.

This will prevent using operations that read or write to the index's on-disk file. This (probably) effectively makes this index indistinguishable from an in-memory index.

This is useful when manipulating index structs, and needing to sever an on-disk index’s link to its associated file. Retcon uses this in its caching architecture.