Move `GTIndexPathspecMatchedBlock` to the top of the file.
GTRepository+StatusSpec.m: Make changes as per review

- Add missing parentheses
- Omit return type
- Pull out method call
Add tests for skipping specific paths and stopping early

Omit block return types and add missing parentheses as well.
No more global state
Update documentation to be more accurate.
Use dot syntax
Style fix
Move ternary to it's own line, pass in nil instead of @""
returnCode -> shouldUpdate