offset index read integration

[Shubham8287]

Description of Changes

1. Create IndexFile: a wrapper around IndexFileMut for exposing only read apis.
2. Update Repo trait to get IndexFile
3. Refactoring: Method naming changes to show clear distinction between read and write version of IndexFile
4. Modify Commits iterator to lookup through Index for initial commit.

API and ABI breaking changes

N/A

Testing

Did some manual testing