Convert git_oid to GTOID before using it in lookupObjectByOid:error:
Add containsObjectWithOID: method and implement containsObjectWithSha:error: using this method.
Add addEntryWithOID:... to GTTreeBuilder and implement addEntryWithSHA:... in terms of this.
Use GTOID class to parse SHA strings.
Add +OIDByCreatingCommitInRepository:... to GTCommit and use that to implement +shaByCreatingCommitInRepository:...
Add +OIDByCreatingTagInRepository:... to GTTag and implement shaByCreatingTagInRepository:... with that method.
Add OID property to GTTreeEntry and use it to implement the hash and sha methods.
Add the OID property to GTOdbObject