Right, `instancetype` is wrong here.

I wasn't sure about how to handle `NSMutableArray` (and since I have a perverse tendency to over-engineer, I'll ask) :
- Is it useful to be able to make a copy from some `libgit2` array and add some things to it ?
- If it isn't, it looks to me we'd be violating some rule by having a `NSMutableArray` method that doesn't actually return a mutable array.
- The best way I could think of would be to subclass `NSArray` and make a real wrapper around `git_strarray` (over-engineering, I said ;-)). This makes us the proud owners of a class-clustered subclass, which is a little scary.

I think I know how to fix it now, but my 3rd point still stands ;-).