Implement SHA256 support as per https://git-scm.com/docs/hash-function-transition/.

High level changes:

• Repository Format Extension
• Packfile
• Support for representing SHA1 and SHA256 objects concurrently.
• Loose object index
• Bidirectional (SHA-1 <-> SHA-256) mapping table
• Fetching from SHA1 server

Operations:

• Init/Open a valid SHA-256 repository (Same for PlainInit/PlainOpen)
• Clone
• Open an existing repository
• Blame
• Signed Commits
• Signed Tags
• Mergetags
• Submodules
• Shallow clone (unsupported - fail gracefully)